By: Katelyn Hansen A2 Essay Example

in rapid light production. Analytical systems typically combine the analytes and reagent in a small chamber directly in front of a PMT to enable almost instantaneous light production. When analytes are eluting from a gas chromatographic column, they are often introduced directly into the reaction chamber at the end of the column.

The aim of the analyst is to maximize the energy used to excite the molecules of the analyte during the reaction. Therefore, losing energy through collisions in the gas phase is undesirable. To minimize deactivation, it is essential to maintain a low gas pressure (~1 torr) in the reaction chamber by using a vacuum pump. It should be noted that determining the specific "products" of the reaction can often be unclear due to the nature and complexity of the reaction itself. In some reactions, chemiluminescent emitters are well-established. However, in this particular reaction, electronically and vibrationally excited HF is identified as the primary emitter. Nevertheless, there are also other unidentified emitters present in this same reaction that contribute to overall light detection by the PMT.

For the analytical chemist, the specific products produced in a reaction are often not important. The main concerns are the instrument's sensitivity (detection limits for target analytes), selectivity (response for an analyte compared to interfering compounds), and linear range of response. The following is a diagram illustrating the necessary components for a gas phase chemiluminescence detector connected to a capillary gas chromatograph.

HISTORY

The term "chemiluminescence" was coined by Eilhardt Weidemann in 1888, and refers to the emission of light from a chemical reaction. It can be represented in its simplest form as follows: Where I* represents an excited state intermediate. This

is known as "direct chemiluminescence".

Indirect chemiluminescence involves the transfer of energy from an excited state to a sensitiser (F) in order to emit light. This phenomenon can occur in gas, liquid, and solid phases, but our focus at Deakin University for Chemical Analysis is on reactions occurring in the liquid phase. Chemiluminescent reactions can vary in terms of intensity, lifetime, and wavelength of emitted light. The range of wavelengths can encompass near ultraviolet, visible light, and even extend into the near infrared spectrum.

Solution-phase chemiluminescent reactions, commonly used for analytical purposes, emit light in the visible region.

WHAT IS IT REALLY?

Chemiluminescence is the release of energy from a chemical reaction, resulting in the production of electromagnetic radiation as light. While the emitted light can fall into either the ultraviolet or visible regions, reactions that emit visible light are more frequently observed and considered more intriguing and valuable. Chemiluminescent reactions can be classified into three types:

1. Chemical reactions involving synthetic compounds, usually including a highly oxidized species like peroxide, are referred to as chemiluminescent reactions.
2. Bioluminescent reactions occur within living organisms such as fireflies or jellyfish and generate light.
3. Electrochemiluminescent reactions produce light through the application of electrical current.

Typically, chemiluminescent reactions involve the breaking or fragmentation of the O-O bond within an organic peroxide compound.

The presence of peroxides, especially cyclic ones, is important in reactions that produce light. This is because the relatively weak peroxide bond can easily break, resulting in a significant release of energy through molecular reorganization. To achieve maximum sensitivity, a chemiluminescent reaction must generate light photons as efficiently as it can. Every chemiluminescent compound or group can produce only one photon

of light. If a reaction were completely efficient, its chemiluminescence quantum yield (pic) would be equal to one.

The chemiexcitation quantum yield (pic) measures the probability of creating an excited electronic state in a reaction, ranging from 0 to 1. A value of 0 represents a completely dark reaction, while a value of 1 indicates that all product molecules are generated in the excited state. A pic value greater than or equal to about 10-3 is considered highly useful for chemiluminescent reactions. On the other hand, the fluorescence quantum yield (pic) refers to the likelihood of the excited state emitting a photon through fluorescence instead of decaying through other processes. This property typically has a minimum value of 0.1 and can range from 0 to 1. Additionally, the reaction quantum yield (pic) represents the fraction of starting molecules that undergo the luminescent reaction rather than a side reaction.

The chemiluminescence yield typically has a value of around 1. However, when the emitter has low fluorescence (), it is possible to increase the yield of chemiluminescence by using a highly fluorescent acceptor. This acceptor transfers the excitation energy from the primary excited state compound to the fluorescent acceptor/emitter. The quantum yield of chemiluminescence can be determined using the equation provided in Chemiluminescent Reactions and Liquid Chromatography. The usefulness of chemiluminescence reactions in detecting compounds in liquid chromatography (LC) heavily relies on post column reactions. For more detailed information about liquid chromatography (including high performance LC), please refer to the link provided.

The main focus of this paragraph is on HPLC chromatographic systems. In High Performance Liquid Chromatography, the injection of liquid phase samples onto

an LC column is done using a syringe and injection valve. The sample is carried down the column by the flowing mobile phase, resulting in chromatographic separation. While most HPLC detectors detect compounds based on physical characteristics like UV absorption, fluorescence, or refractive index difference, chemiluminescence detection is necessary in certain cases. This is especially true when compounds need to be sensitively or selectively perceived, particularly when a target compound must be determined alongside co-eluting compounds that cannot be separated from the analyte.

Chemiluminescence occurs when a chemical reaction produces light, without the need to filter out lamp light for detection. The emitted photons from the analyte molecule can be detected against a black background. This detection method utilizes a photomultiplier, which can detect a significant amount of emitted photons.

An analyte that can be determined through HPLC chemiluminescence is likely to have one of three characteristics: 1) it exhibits chemiluminescence when combined with a specific reagent; 2) it acts as a catalyst for chemiluminescence between other reagents; or 3) it suppresses chemiluminescence between other reagents. Examples illustrating these three characteristics using the well-known luminol reaction are provided below.

Luminol-based chemiluminescence detection involves the reaction of luminol with oxidants like hydrogen peroxide in the presence of a base and a metal catalyst. This reaction produces an excited state product called 3-aminophthalate, which emits light at around 425 nm.

If luminol is the analyte of interest, a post column detector can be designed based on the solution phase reaction of luminol. The schematic shows that one reagent pump delivers a solution containing a dissolved metal ion such as copper(II) or iron(III) to the mixer at the end of the

LC column. The other reagent pump delivers a solution containing the oxidant and a base.

The catalyst used determines both the time required for maximum light emission and the decay profile of that emission. The distance from the mixer to the detection cell is carefully determined to enable sensitive detection, particularly for luminol separated from interfering compounds in the LC column.

In practice, other chemical species can be derivatized using luminol or similar reagents for detection. Another method of detection involves luminol suppression.