In the detection system of amino acid analyzer, post column derivatization is a key step in converting amino acids separated by chromatographic column into derivatives that can be detected with high sensitivity, while fluorescence detection is the core means of amino acid quantitative analysis by capturing the fluorescence signal of derivatives. The combination of the two can solve the problem of weak fluorescence signal and difficult direct high-sensitivity detection of free amino acids, and is the mainstream technology combination for modern amino acid analysis (especially trace analysis). Its principle can be decomposed into two parts: "post column derivative reaction mechanism" and "fluorescence detection optical principle", as follows:
1、 The principle of post column derivatization: converting "difficult to detect amino acids" into "easily detectable derivatives"
Amino acid molecules (excluding tryptophan) themselves lack strong UV absorption or fluorescence emission groups, resulting in extremely low sensitivity for direct detection (unable to meet the requirements of trace analysis at the μ g/L level). The core logic of post column derivatization is to allow amino acids separated by ion exchange chromatography columns (a common separation mode for amino acid analysis) to undergo chemical reactions with specific "derivatization reagents" after flowing out of the column, generating stable derivatives with strong fluorescence (or strong UV absorption), creating conditions for subsequent high-sensitivity detection.
1. Core process: Separation → Mixing → Reaction → Detection
The post column derivatization system consists of a derivatization reagent storage tank, mixer, reaction coil, and temperature control device. The specific steps are as follows:
Amino acid separation: Multiple amino acids in the sample are separated by an ion exchange column (such as sulfonic acid cation exchange resin), and flow out of the column in a specific order due to differences in affinity between different amino acids and the resin;
Reagent mixing: The separated amino acid stream (mobile phase+amino acids) is thoroughly mixed with derivative reagent streams (such as indene ketone, ortho benzaldehyde OPA, 9-fluorenylmethyl chloroformate FMOC) through a "T-shaped mixer";
Thermostatic reaction: The mixed solution enters the "temperature controlled reaction coil" (usually 30-130 ℃, depending on the reagent), where a derivative reaction occurs at a specific temperature to produce fluorescent derivatives;
Entering detection: The reacted derivative flows directly into the fluorescence detector for signal capture.
2、 The principle of fluorescence detection: Capture the fluorescence signal of derivatives to achieve quantitative analysis
Fluorescence detection is based on the phenomenon of "photoluminescence": when a fluorescent derivative is irradiated with light of a specific wavelength (excitation light), the electrons in the molecule absorb energy and transition to high energy levels, then release some energy through non radiative transitions, and then transition back to the ground state and emit light of a specific wavelength (fluorescence). Fluorescence detectors convert fluorescence intensity into electrical signals through "excitation light filtering, fluorescence collection, and signal conversion", ultimately achieving quantitative analysis of amino acids.
1. Core structure and workflow of fluorescence detector
The fluorescence detector mainly consists of a light source, excitation monochromator, sample cell, emission monochromator, photomultiplier tube (PMT), and signal processor. The specific steps are as follows:
Excitation light generation: The composite light emitted by a light source (commonly a xenon lamp, which can provide a continuous spectrum of 200-800nm) is filtered by an "excitation monochromator" (composed of a grating and a slit), retaining only monochromatic light that matches the "excitation wavelength" of the derivative (such as the 340nm excitation light corresponding to OPA derivatives);
Fluorescence excitation and emission: The excitation light passes through the sample cell (flowing through the derivative solution derived from the column), and the derivative molecules absorb energy and emit fluorescence (the fluorescence direction is perpendicular to the excitation light to avoid interference from the excitation light);
Fluorescence filtering and collection: The emitted fluorescence is filtered by an "emission monochromator" (removing stray light and unabsorbed excitation light), retaining only monochromatic light that matches the "emission wavelength" of the derivative (such as the 450nm emission light corresponding to OPA derivatives);
Signal conversion and quantification: The filtered fluorescence is received by a "photomultiplier tube (PMT)", which converts weak light signals into proportional electrical signals (current or voltage). After amplification and analog-to-digital conversion by a signal processor, a "fluorescence intensity retention time" chromatogram is generated;
Quantitative basis: According to the "Lambert Beer law of fluorescence extension form" - within a certain concentration range, the fluorescence intensity of fluorescent derivatives is proportional to the concentration of amino acids. By comparing the "fluorescence intensity of standard amino acid derivatives (standard curve)" with the "fluorescence intensity of sample derivatives", the concentration of each amino acid in the sample can be calculated.
2. The core advantages of fluorescence detection: high sensitivity and high specificity
Compared to UV absorption detection, fluorescence detection has irreplaceable advantages in amino acid analysis, which is also the core reason why post column derivatization is often used in combination with fluorescence detection:
High sensitivity: The detection limit (LOD) of fluorescence detection can reach nmol/L or even pmol/L (1-3 orders of magnitude lower than UV detection), and can detect trace amounts of amino acids in biological samples (such as blood and urine) (such as elevated trace amino acids caused by metabolic abnormalities);
High specificity: It can only detect "derivatives that can produce fluorescence", while the mobile phase and impurities (if not reacted with derivative reagents) have almost no fluorescence signal, which can effectively avoid interference from impurity peaks (such as amino acid analysis after protein hydrolysis in food, without complex pretreatment for impurity removal);
Wide linear range: Within the concentration range of 10 ⁴ -10 ⁵, there is a good linear relationship between fluorescence intensity and amino acid concentration, which is suitable for quantitative analysis of samples with different contents (from trace to constant).
3、 The synergistic logic of post column derivatization and fluorescence detection (why must it be combined?)
The combination of the two perfectly solves the pain point of "separation detection" in amino acid detection, forming a closed loop:
Derivative after separation to avoid interference: Post column derivatization is carried out after amino acids are separated by the chromatographic column, and the derivatization reagent will not react with impurities (such as proteins, salts) in the sample (impurities have been separated by the chromatographic column or do not have amino groups), avoiding "derivative impurities affecting separation" (pre column derivatization may cause peak overlap due to co elution of derivative products and impurities);
Fluorescence after derivatization to enhance sensitivity: By derivatizing "non fluorescent amino acids" into "strongly fluorescent derivatives", fluorescence detection can capture weak signals, meeting the needs of trace analysis (such as precise detection of amino acid metabolism profiles in clinical diagnosis);
The signal is linked to the concentration to achieve quantification: the fluorescence intensity is directly proportional to the concentration of the derivative (i.e. the concentration of the original amino acid), and combined with the standard curve, the content of each amino acid can be accurately quantified with good repeatability (RSD usually<2%).