Total spectroscopy using on-line diode array system

In all areas of water treatment, analysis of the chemical content of process and waste water is a basic procedure. Analysis can be undertaken by using either on-line monitoring instruments or laboratory-based sampling systems, and while both approaches produce accurate results, on-line monitoring also provides real-time results.

In the power generation industry colorimetric analysis has been the accepted method for determining specific analytes in boiler feed water, steam condensate, cooling water and saturated steam for many years. BL-Analytics is proposing that a single instrument combining UV and Vis (visible) spectroscopy could be considered as an alternative approach.

UV spectroscopy

The UV spectroscopy technique involves the application of the complete spectrum range of 200 - 380 nm using a diode array spectrometer. The spectra are stored for a range of samples, with manual reference analysis also being performed on the same samples. These reference data are entered into a chemometrics software package that correlates the spectral response to the reference analysis by the use of various mathematical methods – multiple linear regression, partial least squares or principle component analysis – to derive a calibration for the determinands to which the reference analysis refers.

The samples used to generate the calibration are called the "calibration set" and must cover the whole of the range that is likely to be encountered. A further set of reference analysed samples is then scanned by the system and the results predicted using the newly derived calibrations. These are known as the "validation set" and must not have been included in the calibration.

From the results of the validation it is possible to derive a correlation which should be as close to 1.0000 as possible. However, in practice a correlation between 0.9200 and 0.9999 is acceptable. This technique is widely used in near and mid infrared spectroscopy, but is in its infancy in UV spectroscopy.

Many substances respond to excitation in the UV spectrum, their excitation producing very wide spectra with varying maxima. Compounds such as nitrates respond at two wavelengths, 210 and 225 nm in clean water. In a study of a waste water sample containing nitrate it is likely that the response at these wavelengths will be swamped by other responding organic compounds.

The UV spectrum is affected by the following:

• temperature;

• organic content;

• suspended solids;

• matrix.

It is these factors that have until quite recently restricted the use of UV spectroscopy as a general tool for monitoring organic compounds in water. The technique can also be used to determine non-specific determinands such as COD, BOD and TOC.

Visible spectroscopy

Colorimetric analysis using Vis spectroscopy is the established method used for detecting in water any one or more of the analytes shown in Table 1.

All these methods are very specific and have been designed to tolerate any chemical interferences present. They all require high quality reagents and high quality de-ionised water, especially for silica at very low levels, ie 0-5ppb. The detection limit of all these methods is also critical, so the whole chemistry is carefully optimised.

The systems must also include a calibration system; this is usually performed automatically on a programmable basis every 24 hours.

Analysis system

BL-Analytics have found the DiaMon Resources process system to be effective. The basis of the system is the diode array spectrometer, with a wavelength range of 200-700 nm at 2nm waveband steps. This gives the system the UV (200-380 nm) and Vis (380-700 nm) capability in a single cell. The light source is a flashing xenon lamp. This yields a good continuum over the whole of the UV region.

The sample and reagents are delivered to the flow cell using peristaltic pumps. The reagents are mixed with the sample in the tubing connected to the heater unit and then passed on to the flow cell, a simple quartz glass cell with entry at the base and exit at the top.

The whole operation is controlled by a computer. The untreated sample is fed through the flow cell and the UV scan taken. This is used as the background colour for the following visible analysis and UV analyses.

Much of the work performed has concentrated in the area of detecting the organic contamination of "clean water" and in the determination of BOD5 and COD in final effluent.

The system has been tested for the simultaneous analysis of silica and hydrazine by colorimetry and organic contamination of cooling water and re-circulating waters. In the future, it is likely that the system will be employed for looking into the possibility of using the UV to determine levels of:

• organic oxygen scavengers - DEHA

• presence of oil in cooling systems

• hypochlorite levels in waste water and cooling water.

A series of tests at varying concentrations using hydrazine dichloride showed that there was no response in the UV. But trials have revealed the excellent potential of UV spectroscopy in the rapid determination of hypochlorite levels and the detection of organics. Detecting changes in visible colour in sample streams, some of which could be linked directly to the addition of certain chemicals, is also a possibility.

UV spectroscopy does not replace colorimetric analysis, but adds to the data that can be collected on any sample. There still remains work to be done, but what has been revealed so far indicates the potential of the technique.
Tables

Table 1. Visible spectroscopy standard analytes