by Alexander Dickson

Introduction

WPI's LEDspec is an LED-based, multi-wavelength detector with integrated reference channel. LEDspec is suitable for a variety of applications from nutrient analysis to water purity and trace metal detection.

Partnering with Astoria Pacific, WPI assembled a flow analysis system comprising asn Asotria2, a LEDspec and a liquid waveguide capillary cell (LWCC-2050) with a pathlength of 50cm. 

Equipment Set-Up

Instrument Set-Up

The Astoria2 is set up according to manufacturer’s instructions with some slight modifications. A schematic of the plumbing is shown in Figure 1.

Fig. 1 - Schematic of fluid connections

The overall flow rate into the LWCC is 1.1mL/min, which is consistent with the recommended flow rate for the LWCC. A LEDspec instrument is utilized with a 540nm and 700nm LED module in lieu of the internal light source and detector on the Astoria2 Analyzer. Due to problems with bubble formation in the Astoria Model 311 Random Sampler, an injection valve with a 3.0mL sample loop is used for sample introduction. The internal sample cell is bypassed and the sample line is connected to a LWCC sample cell using 400µm diameter fiber optic assemblies. Data are collected using LEDspec software with a sample rate of 0.2 samples per second and a scan rate of 1.0 samples per second. The 700nm LED module is used for the correction of baseline offset and drift.

Reagent Set-Up

Griess Reagents (NED and Sulfinamide Solutions) are freshly prepared using the following components:

• NED Solution: 0.1% N-1-napthylethylenediaminedihydrochloride in water (w/v)
• Sulfanilamide Solution: 1.0% sulfanilamide in 5% phosphoric acid solution (w/v)

Brij-35 was added to each of the Greiss reagents to a final concentration of 0.06% (w/v). The addition of Brij-35 to the Greiss Reagents facilitates the use of ultrapurified water (18MΩ/cm) as a running solution and thus avoids any baseline shift that may occur during the transition from running solution to sample solution.

Example Experiment:

Experimental Set-Up:

The Astoria2 Analyzer was set-up according to the above recommendations and liquid connections were made to a 50cm sample cell (WPI # LWCC-2050 ). The sample cell was connected to the LEDspec using 1 meter, 400µm diameter fiber optic assemblies (WPI # FO-400-SMA1M ). Nitrite solutions were made gravimetrically in the range of 0.22nM to 1887nM in ultrapurified water (18MΩ/cm) for analysis.

Results:

Intra-sample repeatability of the experiment was excellent with a percent variation of less than 5% for samples in the range of 12.9 to 1887nM. In the 0.22nM and 2.63nM samples, the percent variation was higher, however this increase is expected due to the small signal response and the presence of noise in the system. The linear response including all points had an R² value of 0.992.

 Table 1- Experimental Results

  Fig. 2 - Nitrite experiment with LWCC sample cell

Fig. 3 - Detail of Nitrite experiment with LWCC sample cells showing the lowest three concentrations (0.22nM, 2.63nM, 12.9nM)

Fig. 4 - Plot of absorbance with respect to concentration for experimental results

Fig. 5 - Plot of absorbance versus wavelength

Conclusions

The performance of the nitrite analysis with a LWCC, when coupled with the Astoria-Pacific Astoria2 analyzer and a LEDspec spectrometer, is excellent; both with regard to repeatability and a linear response to concentration.