Search: TOC

α-Tocopherol and α-tocopherol acetate are separated using HPLC in reversed phases and determined with a fluorescence detector or a UV detector.

Total carbon (TC) and total inorganic carbon (TIC) are converted into carbon dioxide through oxidation (TC) or acidification (TIC). The CO₂ is transferred from the digestion cuvette through a membrane into the indicator cuvette. The color change of the indicator is measured in a photometer. TOC (total organic carbon) is determined as the difference between the values for TC and TIC.

The organically bound carbon in water is converted into carbon dioxide through combustion. Inorganic carbon is eliminated through acidification or is determined separately. The carbon dioxide formed by oxidation is determined either directly or after reduction, e.g., to methane. The final determination of CO₂ is achieved using various methods, e.g., infrared spectrometry, CO₂-sensitive sensors or – after reduction of the CO₂ to yield methane – with a flame ionization detector.

The basis for these O₂ measurements is the detection of photoluminescence produced by an oxygen-sensitive layer. The change in photoluminescence depends on the partial pressure of the oxygen. Given the values for the partial pressure of the oxygen and the temperature, the amount of oxygen gas dissolved in the liquid can be calculated. The oxygen sensor determines the O₂ content of the liquid by means of optical detection through a photoluminescent process, in which an oxygen-sensitive layer is exposed to blue light. In doing so, the molecules in this layer become excited and reach a higher energy state. In the absence of oxygen, the molecules emit a red-colored light. If oxygen is present, it collides with the molecules in the oxygen-sensitive layer. The molecules in the oxygen-sensitive layer, which have collided with oxygen, cease to emit light (refer to figure 1). For this reason, a relationship exists between the oxygen concentration and the intensity of the emitted light as well as the intensity and the rapidity with which the intensity of the light diminishes. The intensity of the light is reduced at higher oxygen concentrations, although the rate at which it does so increases. The temperature of the product and the time interval between the light signal and the emission of light (phase shift) are both measured and used to calculate the oxygen content.

The device’s construction enables the state of the blue LED to be monitored using a photodiode. Another photodiode – with a red filter – measures the oxygen-dependent red light (refer to figure 2). This light is emitted by the luminophores due to photoluminescence (fluorescence) after they reach an excited state through exposure to the blue light. As a result of this exposure, the electrons of the luminophores are elevated to a higher energy level. As they return to their original energy level, they emit a red light.

The CO₂ content is determined as inorganic carbon using a device that measures the total organic carbon, known as a TOC analyzer. Through the addition of caustic, CO₂ in the sample enters the system as carbonate or bicarbonate and then through the addition of phosphoric acid is released and fed into an NDIR detector (non-dispersive infrared sensor) by means of a carrier gas.

Incubators [1, 2]

Choosing the right temperature range when incubating microorganisms is crucial for successful detection. The optimum temperature, i.e. the temperature at which the organism grows at the maximum rate, is often only a few degrees below the maximum temperature at which the cells start to become damaged.

In microbiological practice, we mostly deal with mesophilic organisms, i.e. organisms that have an optimum temperature between 20 and 45 °C.

Conventional incubators [1, 2] allow incubation up to 80 °C. The following requirements/instructions should be taken into account:

• Do not choose an incubator that is too small

• The incubator should not be filled too tightly in order to achieve good air circulation and uniform heating

• The interior should be easy to clean

• The material in the interior should preferably be stainless steel (smooth surfaces, rounded corners, removable, tilt-proof inserts)

• An inner door made of glass is recommended for observing the cultures to avoid frequent opening of the doors

• The culture media must not be allowed to dry out, otherwise the results may be distorted. Devices with natural convection are therefore ideal, as drying out is not accelerated, in contrast to devices with forced convection

DIN 12880:2007-05 applies to the testing of heating ovens and incubators in Germany. This standard specifies the measurement setup for determining temperature homogeneity and temperature consistency, for determining heating and cooling times and for determining recovery times after opening the door. The setting accuracy of modern incubators should be 0.1 °C.

If the temperature exceeds or falls below the target temperature, this should be signalled by a visual or audible alarm.

Nevertheless, temperature differences can occur inside incubators, especially when fully loaded. It is therefore advisable not to read the actual incubation temperature on the control thermometer of the incubator, but on thermometers placed in water-filled flasks/bottles and positioned inside the incubator.

Incubators with a CO₂ atmosphere or cooled incubators are also available for special applications. While normal incubators are operated at room temperature and above, cooled incubators are also suitable for cooling down to temperatures as low as 0 °C.

Refrigerators/freezers

Many companies use standard household refrigerators to store culture media solutions, culture plates, etc.

Appliances specially designed for laboratory use have a number of additional features:

• Lockable doors

• Easy-to-clean interiors and smooth doors without storage options

• Acoustic and visual temperature and door opening alarm

• Integrated data memory for temperature recording

• Interface to read data

• Higher temperature consistency and stability than household appliances

• Digital temperature display

• Option to route external temperature sensors

Should it be possible for an explosive atmosphere to develop inside, refrigerators with an explosion-proof interior must be used (in accordance with the ATEX 95 directive or BG-I 850-0)

Several user-friendly systems are available for microbiological strain preservation by means of deep freezing (cryopreservation). Storing cultures in a frozen state at very low temperatures guarantees a high genetic stability of the cells and good consistency of their characteristics over a longer period of time.

Storage in household freezers is definitely not recommended. For this purpose, special and comparatively expensive ultra-low temperature freezers are used, which can cool down to temperatures < -70 °C. At temperatures around -20 °C, the death rate of many microorganisms is many times higher than at -70 °C.

Laboratory dishwashers [1, 3]

The term laboratory dishwasher is the colloquial term for "washer-disinfector". These have to fulfil the special requirements of the laboratory environment and have considerably more complex technical equipment than household dishwashers, which is reflected not least in the price.

Here are some examples of the differences to conventional dishwashers:

• Use of special industrial detergents that are more aggressive than household detergents

• Double-walled and insulated, washing chamber in 1.4404 grade stainless steel

• Special inserts and spray nozzles designed for laboratory materials

• Safety interlock

• More cleaning programmes

• Antibacterial thermal disinfection up to 95 °C

• Dosing pumps for liquid detergent and neutraliser

• Integrated water softener with salt sensor

• Cold water connection and connection for fully demineralised water for rinsing

• Conductivity monitoring to ensure residue-free operation

• Interface for PC or external printer

Autoclave [1]

In microbiology, culture media and consumables often have to be sterilised before they can be used. The choice of procedure depends on the properties of the item to be sterilised, its resistance to the active agent and the type and extent of contamination. If possible, heat sterilisation should be carried out in a final container that is safe from recontamination, whereby autoclaving offers the greatest safety.

Autoclaving involves sterilisation with moist heat, i.e. with pressurised saturated water vapour. The temperatures required to kill endospores (approx. 120 °C) are reached at vapour pressure values above atmospheric pressure.

The main component of the autoclave is a pressurised vessel that can be tightly sealed with a lid. The lower part of the vessel is filled with water, which is vaporised by an electric heater.

Autoclaves are available in different designs (vertical, pot or horizontal autoclaves). There are also single-walled and double-walled autoclaves. Double-walled autoclaves can be used more universally and the sterilised items can be removed dry, as the steam vapours can be removed using a vacuum pump.

Autoclaves are subject to the Pressure Vessel Directive and must be inspected by experts at regular intervals.

Dry steriliser [1]

Hot air sterilisation processes are primarily used to sterilise heat-resistant instruments as well as laboratory, glass and appliance parts. Proteins are denatured much more easily in a moist environment than in a dry state, which is why higher temperatures and longer exposure times are required for hot air sterilisation processes. The appliances work with either natural or forced convection. In the latter case, a fan is used to achieve faster heat transfer and therefore a temperature that is as consistent as possible. Due to the low heat capacity of air, the heat transfer takes considerably longer and depends on the packaging and the weight of the goods.

Common temperatures and timings are as follows: