Analysis of elements

With the ‘continuous flow auto-analysers’ from General Instrumentation, elements in water and soil samples are analysed for biogeochemical-ecological research. The content of PO4 (phosphate), NH4 (ammonia), NO3 (nitrate), NO2 (nitrite), Cl (chloride) is determined colorimetrically. The parameters Na (sodium) and K (potassium) are determined by flame photometry.

Of the five auto-analysers (model III systems or AA500, three systems from Bran & Luebbe and two from Seal) that are on the GI, a number are from the spin-off company B-ware and others from the GI itself. The collaboration between Radboud University and this company is also reflected in the staffing: Sebastian Krosse of B-ware has been seconded as chief analyst, and Paul van der Ven of the GI is responsible for elemental analysis at the GI.

The full description of each car analyser can be found in the booking system.

ICP-OES (Inductive Coupled Plasma-Optical Emission Spectrometry) is a robust, high-throughput technique that uses plasma and a spectrometer to determine the composition and concentration of elements in a liquid.

The GI and B-WARE have three ICP-OESs, respectively an ICP-OES iCAP 6000 (Thermo Fischer Scientific; Bremen, Germany), an ICP-OES ARCOS MV and a GREEN (Spectro Analytical; Kleve, Germany). The description of each ICP-OES can be found in the booking system.

The solution to be analysed, which is usually watery, is fed into the ICP-OES by means of a peristaltic pump and atomised into an aerosol in a chamber. This aerosol is then brought into an argon plasma using a stream of argon gas. Plasma is generated at the end of a quartz torch using a cooled induction coil through which a high-frequency alternating current is sent. This creates an alternating magnetic field that accelerates the electrons and causes them to move in a circular path. Collisions between argon atoms and electrons cause ionisation, which maintains the plasma.

The plasma is very hot, around 6000-7000K, and even up to 10000K in the induction zone. The torch extracts the solvent and atomises and ionises the sample. The added thermal energy causes the electrons to enter a higher, ‘excited’ state. When the electrons fall back to their ‘ground state’, energy is released in the form of light, ‘photons’. Each element has a characteristic emission spectrum. Spectral properties and light intensity are measured using detectors. The concentration of the elements present can be calculated using this data.

ICP-MS (inductively coupled plasma-mass spectrometry) is a technique for determining trace (ppb level = µg/l) and ultra-trace elements (ppt level = ng/l) in which atomic elements are converted by a plasma source into ions that are then separated based on their mass. The advantages of the ICP-MS technique compared to AAS (Atomic Absorption Spectroscopy) or ICP-OES (inductively coupled plasma optical emission spectrometry) are:

• Extremely low detection limits
• A large linear range
• The ability to determine the isotopic composition of the elements 

In addition, as with ICP-OES, the technique has a multi-element character and a high sample throughput. Disadvantages of an ICP-MS are the occurrence of spectral and non-spectral interferences and the high cost. The GI's ICP-MS is the ICAP Q model from Thermo Fisher Scientific.

A Vario Micro Cube (Elementar) is used to analyse the carbon, nitrogen, hydrogen and sulphur content in solid samples.

Method

Solid matter is weighed in a tin container and placed in an autosampler. From the sampler, the sample falls into a combustion tube, where it burns in a flash (flash combustion) at 1800 °C under the influence of oxygen and the tin. The combustion gases are passed over a copper oxide column with the aid of helium as a carrier gas, so that they are completely converted into CO₂, N₂, NO_x, H₂O, SO2 and residual O₂.

These gases then flow through a copper column, where nitrogen oxides are reduced to elemental nitrogen and O₂ to CuO. The water can then be absorbed by a column. The gases go to a TPD (Temperature Programmed Desorption) column where CO₂, H₂O and SO₂ are absorbed and the gases are released one after the other with the help of programmed heating. They flow through a thermal conductivity detector (TCD), which emits an electrical signal proportional to the concentration of nitrogen, carbon, hydrogen and sulphur.

More information can be found in the booking system.

Stable isotopes can be measured by linking the CN elemental analyser to a Thermo Fisher Scientific, Delta V Advantage IRMS.

Release of N2 and CO2 in the CN elemental analyser
 

In the first step, samples consisting of solids (plant tissue, animal material, soil, sediment, etc.) are very quickly and quantitatively converted into combustion gases of the elements of which they are composed. The products of this separation are mainly pure N2 and CO2. This conversion takes place in the CN elemental analyser. The sample preparation for radioisotopes is similar to that for C and N analysis, but there are specific requirements.

Isotope separation in the IRMS

The δ N and δ C are then determined. This is done in the IRMS. Products from the CN elemental analyser are fed into the MS, where they are ionised, accelerated and separated. At the end of the separation process, these ions are collected by special detectors (Faraday cups), three of which are positioned in such a way that three masses can be detected simultaneously: for nitrogen (N2) these are masses 28, 29 and 30, and for carbon (CO2) masses 44, 45 and 46.

Dual isotope analysis
 

In this analysis, the C:N ratio must be such that both nitrogen and carbon can be optimally measured, but often the carbon content is too high. If necessary, the CO2 signal in the Conflo can be diluted with an extra helium flow so that the CO2 content also falls within the correct measuring range. With a high C:N ratio, you have to be careful that the signal of masses 28 and 29 of nitrogen does not become too low.

More information can be found in the booking system.

The General Instrumentation has a Methrom 930 Compact IC Flex ion chromatograph with autosampler. It is mainly used to measure and separate anions based on charge in water samples. 

In this process, 3 ml of water is injected onto a special column that separates the anions based on charge, measuring from fluoride, chloride, phosphate, nitrate to sulphate. Succinate is also possible in a different programme. For more questions, please contact Sebastian Krosse.

The GI has two Shimadzu GCs for measuring either H2S or CH4. The methane GC is equipped with an autosampler for different types of sample tubes and is only used for headspace measurements. Special 20ml vials must be used for hydrogen sulphide measurements. More information can be found in the booking system.

The Total Organic Carbon analyser present at the GI facility is a TC-TNb instrument from Analytik Jena. It can be used to measure the (bound) carbon and nitrogen content in liquids (e.g. river and seawater) and sludge (e.g. from bioreactor extract).

More information about sample preparation and the measurement itself can be found in the booking system.

Two ball/vibrating grinders are available to aid in elemental analysis, with which small amounts of material (up to 8 mm) can be ground into a homogeneous powder (final size 0.005 mm). These devices make a lot of noise and generate fine dust. Therefore, they should always be used in the designated protective unit. Before using the machine, ask the manager how to operate the bullet grinder, which holders (metal or Teflon) and bullets to use, how to adjust the settings and how to clean it properly.

The two bullet grinders available have different purposes:

• The MM400 is for non-enriched material only.
• The MM310, only for enriched material
   

Two microbalances (Sartorius) are available to users to weigh CN or IRMS samples that have already been ground to a fine powder with a ball mill. The quantities of material are in the range of 0.1 to 40 mg. It is important to work with extreme cleanliness, especially when working with stable isotope samples. Consult with the manager about weighing attributes and reserve the balances well in advance via the booking system.