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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.

Dissolved nitrogen in a liquid medium is measured using the same procedure as the CO₂ determination, i.e., using heat conductivity.

CO₂ is employed as a purge gas in the beverage industry. Therefore, in order to measure nitrogen, the change in thermal activity and CO₂ and N₂ is used. The thermal conductivity is determined in a small measurement chamber, which in turn is separated from the material being measured by a semipermeable membrane. Diffusion through the membrane changes the thermal conductivity in the measurement chamber.

The gas volume in the measurement chamber is fully replaced in cycles of 10–20 s. The changes in thermal conductivity over time are a measure of the diffusion of N₂ through the membrane, which allows the concentration in the medium to be calculated, taking temperature into account.

The calculation for the concentration of N₂ is achieved using the change in thermal conductivity in the measurement chamber, also taking the temperature into account.

Since the thermal conductivity of oxygen is similar to that of nitrogen, a second channel may need to be used to compensate for any oxygen in the medium [1].

The analytical determination of oxygen using amperometric electrodes is achieved through measurement of the electrical current. The electrodes consist of a cathode and an anode, which are connected conductively through an electrolyte solution (KCl/KOH). Precious metals, such as platinum and gold are chosen for the cathode, and silver, for the anode. The gas-permeable membrane separates the two electrodes from the solution being measured. An appropriate polarization voltage causes diffusion of the oxygen across the membrane into the measurement cell, where it reaches the surface of the cathode and is reduced, producing hydroxide ions.

Reaction at the cathode:     O₂ + 4e^– + 2 H₂O      → 4 OH⁻

Reaction at the anode:        4 Ag + 4 Cl⁻               → 4 AgCI + 4e⁻

This chemical reaction creates an electrical current that is proportional to the partial pressure p_O2 of the oxygen. Oxygen must be steadily liberated from the solution being measured for the oxygen electrode to receive a constant supply. The concentration of oxygen in the medium can be determined using HENRY’s law and the solubility coefficient of oxygen [1]. Three different variations on the types of the equipment required for performing this analysis are present below.

Osmolality is defined as the number of particles of osmotically active substances per kilogram of a solvent (usually water). The size or type of particles is irrelevant for the osmotic pressure, only the number of particles (cations, anions, sugars, organic acids, amino acids, proteins, ethanol, etc.) is of importance. The presence of substances dissolved in an aqueous solution lowers the freezing point, as compared with pure water. The freezing point is lowered in proportion to the amount of dissolved particles or molecules. For this reason, measuring the freezing point of a solution provides a means for calculating the concentration of dissolved particles. The more dissolved particles there are in a solution, the greater the drop in freezing point.

Glucose is phosphorylated by the enzyme hexokinase (HK) and adenosine 5'-triphosphate (ATP) to glucose 6-phosphate (G-6-P)

\(\text{Glucose + ATP} \space ^{\underrightarrow{\text{HK}}} \space \text{G-6-P + ADP}\)

In the presence of the enzyme glucose-6-phosphate dehydrogenase (G6P-DH), G-6-P is oxidized by nicotinamide adenine dinucleotide phosphate (NADP) to gluconate-6-phosphate. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) is formed:

\(\text{G-6-P + NADP} \space ^{\underrightarrow{\text{G6P-DH}}} \space \text{gluconate-6-phosphate + NADP + H}^+\)

The amount of NADPH formed during the reaction is equivalent to the amount of glucose. NADPH is determined based upon its absorbance at 334, 340 or 365 nm.

Glucose and fructose are phosphorylated by the enzyme hexokinase (HK) and adenosine 5'-triphosphate (ATP) to glucose 6-phosphate (G-6-P) and fructose 6-phosphate (F-6-P):

Glucose + ATP \(^{\underrightarrow{HK}}\) G-6-P + ADP

Fructose + ATP \(^{\underrightarrow{HK}}\) F-6-P + ADP

In the presence of the enzyme glucose-6-phosphate dehydrogenase (G6P-DH), G-6-P is oxidized from nicotinamide adenine dinucleotide phosphate (NADP) to gluconate-6-phosphate. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) is formed:

\(\text{G-6-P}\hspace{0.2em}+\hspace{0.2em}\text{NADP}\hspace{0.8em}^{\underrightarrow{\text{G6P–DH}}}\hspace{0.8em} \text{glucanate-6-phosphate} + \text{NADP}+\text{H}^+\)

The amount of NADPH formed during the reaction is equivalent to the amount of glucose. NADPH is a measurand and is determined based on its absorbance at 334, 340 or 365 nm.

After the reaction is complete, F-6-P is converted to G-6-P by phosphoglucose isomerase (PGI):

F-6-P \(^{\underrightarrow{PGI}}\) G-6-P

G-6-P reacts in turn with NADP to form gluconate-6-phosphate and NADPH. The additional amount of NADPH formed is equivalent to the amount of fructose and is determined photometrically based on its absorption at 334, 340 or 365 nm.