The Effect of Needle Gauge and Flow Rate on Evaporation Rate

Sample preparation is one of the most time-consuming steps in analytical workflows. Whether a lab is conducting forensic analyses, environmental testing, pharmaceutical development, or food safety analysis, researchers often face the challenge of concentrating samples before analysis. Nitrogen blowdown evaporation has become a widely used technique for this purpose because it is simple, efficient, and adaptable to a wide range of solvents. When preparing samples for evaporation, factors that play a major role in efficiency include: needle gauge and flow rate. Optimization of these parameters could aid in developing more effective solvent evaporation methods.

At Organomation, all blowdown evaporators come with standard 4-inch, 19-gauge stainless steel needles. This size has proven to be effective for most laboratory applications as it balances evaporation speed with nitrogen consumption while also accommodating for common sample tube sizes. For labs that need to process higher sample volumes, 16-gauge and 14-gauge needles are also available. All needles are constructed from 304 stainless steel and feature a luer lock fitting for easy attachment and removal from the nitrogen evaporator.

Traditionally, Organomation has recommended using a straight 19-gauge needle with a flow rate of 0.33 L/min per sample. This setting works well for most solvents, but how could changing these parameters aid in increasing evaporation efficiency? 

- Could switching to a larger gauge needle accelerate evaporation? 

- Does increasing nitrogen flow rate always improve efficiency, or are there trade-offs? 

- Can needle design modifications reduce splashing while maintaining faster drying times? 

To investigate this, a series of controlled trials using methanol as a representative solvent were conducted. The study compared evaporation rates across different needle gauges, flow rates, and needle shapes to provide practical insights for laboratories seeking to optimize their sample preparation methods.

Needle Variations

While straight stainless-steel needles are the standard for nitrogen blowdown evaporation, alternative needle shapes may influence how nitrogen interacts with the solvent surface. Two main modifications were tested in this study:

- Bent needles: Designed with either a short bend or a long bend near the end of the needle. The goal was to change the angle of nitrogen delivery to reduce splashing and improve solvent exposure. 

- Pseudo-spiral needles: Created by adding two bends to the bottom quarter of 19-gauge needles, forming it into a spiral-like curve. These were intended to generate a more turbulent nitrogen flow pattern, potentially enhancing evaporation. 

Both designs were hypothesized to improve evaporation efficiency by concentrating gas flow while minimizing sample loss.

Experimental Design

The study evaluated how needle gauge, needle modifications, and nitrogen flow rate influence methanol evaporation under controlled conditions. All experiments were conducted at both room temperature and 63 °C, the recommended bath temperature for methanol.

Needle Gauge

- Compared 19-, 16- and 14-gauge needle evaporation rates. 
- Used each needle’s recommended flow rate 

- 19 ga: 0.33 L/min
- 16 ga: 0.83 L/min
- 14 ga: 1.2 L/min

- Purpose: determine whether larger needle diameters accelerate evaporation relative to the standard 19-gauge configuration. 

Needle Modifications

- Compared straight needles to bent and pseudo-spiral designs. 
- 19- and 16-gauge needles were modified to have a short bend or long bend on the end of the needle. 
- Additional pseudo-spiral designs of the 19-gauge needles were created by bending the lower quarter of the needle into two spiral shapes. 
- Purpose: evaluate whether altering gas flow direction could improve evaporation rates. 

Flow Rate Variation

- Increased the nitrogen flow rate past original recommendations.
- Used the recommended flow rate for 16-gauge needles on the 19-gauge bent needles.
- Used the recommended flow rate for 14-gauge needles on the 16-gauge bent needles.
- Purpose assess whether higher flow rates improve efficiency and if pairing higher flow rates with bent needles can reduce splashing. 

Results

Table 1. Different Needle Sizes - Methanol Evaporation Rate

The 14-gauge had the fastest evaporation rate, which was expected, as the standard flow rate for this size needle is the fastest. All needle sizes showed an increase in evaporation when using the heated water bath.

Table 2. 19-gauge Needle - Methanol Evaporation Rates

Table 3. 16-gauge Needle - Methanol Evaporation Rates

The most effective needle shape for both the 19- and 16-gauge needles, without increasing nitrogen flow rate, proved to be the standard straight needles for both room temperature and heated water bath evaporation. There was a significant jump observed in the heated evaporation, especially in the 16-gauge needles, with over 75% increase. When comparing the short and long bend needle configurations, there were no significant differences, as the style with faster evaporation varied between trials.

Table. 4 Methanol Evaporation Rates with Increased Flow Rates

At room temperature, bent needles showed only a slight improvement in evaporation rates compared to larger gauge straight needles at the same flow rates. However, under heated water bath conditions, needle gauge size played a more significant role. The larger-gauge straight needles achieved substantially higher evaporation rates than bent needles at comparable flow rates. This indicates that gauge size and flow rate have a stronger influence than needle shape when a heated water bath is used.

Key Takeaways

- 19-gauge needles remain reliable, especially at higher-than-standard flow rates. 

- Flow rate is critical: higher flow rates generally increased evaporation speed, but also introduced challenges such as potential sample splashing 

- Heated baths amplify overall performance: all needles performed significantly better at 63 °C water baths than at room temperature. 

- Needle design benefits: bent needles showed potential in minimizing splashing. They allowed for increased flow rate and for the needle to be closer to the sample solution inside the tube. 

Applying These Findings in Your Lab

The results of this study show that needle gauge, flow rate, and needle design can all significantly influence evaporation speed. But how do these findings translate into day-to-day laboratory use?

- Stick with the standard setup for most workflows. For routine sample preparation, the 19-gauge needle at 0.33 L/min remains an ideal choice, offering reliable performance while minimizing nitrogen consumption. 

- If your lab works with high-volume samples or needs quicker turnaround, switching to a 16-gauge needle and increasing the flow rate from 0.83 L/min to 1.2 L/min could provide significant time savings especially when paired with a heated water bath. 

- Faster nitrogen delivery can improve speed, but it also increases the risk of sample loss. Start by slowly increasing flow rate, and then adjust needle position above the solvent surface to control splashing. 

- Consider using bent or spiral needles for reducing splashing while maintaining strong evaporation rates. While not always necessary, they may be helpful in some scenarios. 

Conclusion

For most applications, the standard 19-gauge, 0.33 L/min setting remains sufficient. However, labs working with high-volume samples or those needing faster evaporation can benefit from using 16-gauge needles at higher flow rates, especially when paired with a heated water bath. Needle modifications can help address the issue of sample splashing, although they have shown to decrease the rate of evaporation when compared to the standard straight needles of the same gauge. Ultimately, the choice of needle gauge and flow rate should balance speed, safety, and solvent conservation. These results provide a framework for labs looking to optimize their evaporation setup.

Contact our team to discuss the best needle option and gas parameters for your application.