29 May 2019

Analysing Compounds in Groundwater | FTIR Spectroscopy

With natural potable water exposed to more and more contamination by micro-pollutants from industrial, agricultural and even natural origin, the monitoring of water quality is more important than ever. Groundwater analysis is an important part of this multi-spatial and multi-temporal problem, ranging from surveillance monitoring of surface or groundwaters to operational monitoring of wastewaters, before after treatment.

diagram of water table

The continuing success of industries such as brewing and distilling and agriculture depends upon their access to the clean filtered water from aquifers. It is this water that could be prone to contamination from fertiliser runoff or even due to the consequences of industrial activity such as fracking.

Organisations such as the World Health Organization (WHO), Environmental Protection Agency in the US and European Union provide guidelines for the maximum allowable concentrations of various contaminants in water. There is a multitude of ways to measure contaminants but one of the emerging methodologies is based upon Fourier Transform infra-red technology (FTIR). Analysis involves taking samples, pressing into pellets and performing FTIR measurements.

Water quality

A range of tolerances are used to describe water-quality. These can be generally subdivided into: 

  • indicator
  • microbiological
  • organic (disinfectants and pesticides)
  • and inorganic
This means the concept of water quality is fundamentally a multivariate one. Examples of indicator parameters are colour, conductivity, pH, odour,and taste. Organic chemicals in natural waters can include chlorinated alkanes, benzenes, and phenols. Inorganic chemicals include heavy metals such as lead, nickel or mercury.

Microbiological issues can include Escherichia coli (E. coli), Enterococci and Pseudomonas. FTIR has been used in water analysis for the detection and evaluation of organic and inorganic components. However, more recent research has also seen FTIR with multivariate analysis used to successfully screen water for Pseudomonas aeruginosa and E. coli, which are an indicator of general water quality and the presence of mammalian faeces respectively.

FTIR and the analysis of water

FTIR uses the mid-infrared range of radiation (3333–200 cm−1) to provide spectra with a huge amount of data on the molecular components of both inorganic and organic material. Transmission-FTIR actually follows the linear Lambert-Beer law relationship between compound concentration and absorbance within an acceptable range. This makes FTIR useful for quantitative measurement of heterogeneous samples allowing quantities of inorganic, organic and organo-mineral compounds to be measured.

Although, FTIR is still a relatively insensitive technique and is not suitable for the quantitative detection of low-level organics at the parts per billion level (this would probably use gas chromatography-mass spectrometry (GC-MS)). However, quantitative FTIR spectroscopy has become useful in the assessment of groundwater even though multi-component samples might contain overlapping signals (limit of detection in the parts per million range).

The signals can generally be resolved by using curve resolution techniques such as Positive Matrix Factorization (PMF) to isolate the underlying spectra of single compounds. This method (transmission FTIR) has been used to examine colloids composed of minerals and organics to give an accurate determination of their concentration and quantity in freeze-dried samples.

In addition. FTIR has been used to examine the water structure of K2SO4and MgSO4solutions. By using FTIR- attenuated total reflectance changes in water structure can be detected as influenced by any dissolved salts. FTIR ATR spectra show that MgSOis a water structure making salt, and K2SO4is a structure breaker. Following subtraction of the O–H band spectrum of salt solutions from the pure water reference, the changes in the ‘ice-like’ structure of water influenced by dissolved salts can be determined even at low concentrations.

The presence of minerals such as magnesium ions can change the behaviour of colloids in water. The water structure making effect of the Mg2+ion, decreases the mobility of water molecules results in poor dispersion of the collected precipitate. 

Specac and FTIR preparation

FTIR is an important technique that can be used in the analyses of compounds in a wide range of applications. One of these is the analysis of organic-inorganic complexes in water. The importance of FTIR, however, can only be fully realised if samples for analysis are prepared well as FTIR can be very technique sensitive. For transmission FTIR there are several options.

One of these is the KBr or in the case of the colloid analysis, KBr doped with a known quantity of potassium ferrocyanide as an internal standard. KBr is essentially invisible to FTIR. Specac provides a microprocessor controlled automatic hydraulic press to produce and release KBr disks of consistent thickness and quality. The reproducibility of FTIR analysis methods relies upon the reproducibility provided by Specac presses and dies and good technique. The computer control of the Specac hydraulic presses is a real bonus.

Some FTIR water analyses are not compatible with KBr discs and transmission and in these cases attenuated total reflectance (ATR) methodology can be used. Specac’s Golden Gate ATR is a single reflection monolithic diamond ATR sampling accessory with interchangeable ZeS and KRS-5 lenses for different applications. This accessory features a Type IIIA diamond ATR element metal-bonded into a tungsten carbide mount to give it robustness and dimensional stability.

golden gate atr

The Golden Gate ATR accessory is ideal to examine ‘ice’ samples of water containing dissolved salts to determine their influence on the water structure. Of course, ATR has such versatility that with set quantities of internal standards and good data processing the quantity of a range of water constituents can be determined from lyophilised samples.

References

  1. Andreas Fritzsche, Thomas Ritschel, Louis Schneider, Kai U.Totsche, Identification and quantification of single constituents in groundwater with Fourier-transform infrared spectroscopy and Positive Matrix Factorization, Vibrational Spectroscopy, Volume 100, January 2019, Pages 152-158
  2. Fangqin Cheng and Qinbo Cao et al., FTIR analysis of water structure and its influence on the flotation of arcanite (K2SO4) and epsomite (MgSO4·7H2O), International Journal of Mineral Processing, Volume 122, 10 July 2013, Pages 36-42
  3. Pejcic, B., Myers, M. and Ross, A. (2009) Mid-Infrared Sensing of Organic Pollutants in Aqueous Environments. Sensors 9(8), 6232-6253
  4. Graveline, N., Maton, L., et al., An operational perspective on potential uses and constraints of emerging tools for monitoring water quality. TrAC Trends in Analytical Chemistry 29(5), 378-384
  5. Paula, Peter Atkins, Julio de (2009). Elements of physical chemistry (5th ed.). Oxford: Oxford U.P. p. 459. ISBN 978-0-19-922672-6.

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