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Amino acids are the building blocks of proteins and play a central role in almost all biological processes. In the human body, they are essential for the formation of tissue, the repair of cells and the production of enzymes and hormones. There are 20 different amino acids, some of which are labelled "essential" because the body cannot produce them itself and they must therefore be obtained from food. Amino acids are also crucial for the metabolism, the immune system and the regulation of numerous physiological functions, such as the transport and storage of nutrients.
Amino acid analysis is therefore of great importance as it provides precise information on the quantity and profile of amino acids in biological samples, food or pharmaceutical products. These analyses are essential for assessing nutritional status, diagnosing metabolic disorders and ensuring the quality of food and food supplements. In medicine, amino acid analysis helps to detect imbalances that may indicate health problems, such as liver disease or genetic metabolic disorders. In the food industry, it contributes to the control and optimisation of protein content, which is particularly important for quality assurance and compliance with legal requirements.
Overview of the different amino acids and the polarity of the side chains.
In order to detect amino acids using fluorescence or to separate them using the reversed phase, amino acids must be derivatised. The classic method uses separation of the amino acids by ion exchange chromatography with subsequent post-column derivatisation to enable detection with a fluorescence detector. Ortho-phthaldialdehyde (OPA), 9-fluorenyl-methoxycarbonyl-chloride (FMOC-Cl), ninhydrin or fluorescamine can be used as reagents.
The most modern technique for the derivatisation of amino acids is pre-column derivatisation with ortho-phthaldialdehyde (OPA) and subsequent separation using reversed phase chromatography and subsequent fluorescence detection. The advantages here are the fast analysis times and the high sensitivities. However, a major disadvantage is that OPA only reacts with primary amines. This derivatisation method does not work for secondary amines such as proline.
The reagent 9-fluorenyl-methoxycarbonyl-chloride (FMOC-Cl) is therefore used for the pre-column derivatisation of secondary amines. After this derivatisation, the amino acids can also be separated using reversed-phase chromatography and detected using fluorescence.
Additional hardware is required for post-column derivatisation. The derivatisation reagent must be added to the amino acids in precise doses after the separation of the amino acids. This is achieved using an additional pump after the separation. After the reagent has been added, the amino acids need a certain amount of time to react. Therefore, a loop or a specially woven capillary is used to obtain a specific reaction time. An example setup can be found in the figure on the right.
An on-line solution can also be implemented for pre-column derivatisation. For this, only the arrangement of the post-column derivatisation has to be changed so that the derivatisation takes place before the column. However, this derivatisation can be carried out off-line, which means that there is no additional hardware requirement for the HPLC system. The sample is already derivatised before it is injected into the HPLC system.
It is also possible to separate and detect the amino acids without derivatisation. As in post-column derivatisation, the separation takes place using ion exchange or other special columns. The subsequent detection is then carried out using mass spectrometry (MS). The Intrada Amino Acid from Imtakt should be highlighted here as a special column for the separation of non-derivatised amino acids. With these columns it is possible to analyse the 20 amino acids within 10 minutes with MS/MS detection without derivatisation.
Determination of 20 amino acids
Trp = tryptophan Phe = phenylalanine Tyr = tyrosine Met = methionine Leu = leucine , Ile= isoleucine Val = valine Glu = glutamic acid Pro = proline Thr = threonineAsp = aspartic acid Ala = alanine Ser = serine Gln = glutamine Lys = lysine Gly = glycine Asn = asparagine Cys = cysteine His = hystidine Arg = arginine
Column: Intarda Amino Acid 50 x 3 mm
Mobile phase A: acetonitrile /tetrahydrofuran /25mM ammonium formate /formic acid = 9 /75 /16 /0.3
Mobile phase B: Acetonitrile /100mM ammonium formate = 20 / 80
Gradient:
Time (min) | %B |
0 | 0 |
2.5 | 17 |
6.5 | 100 |
Flow: 0.6 mL/min
Temperature: 35°C
Injection volume: 5 µL
1. OPA-Aspartic Acid 2. OPA-Glutamic Acid 3. OPA-Asparagine 4. OPA-Serine 5. OPA-Glutamine 6. OPA-Histidine 7. OPA-Glycine 8. OPA-Threonine 9. OPA-Citrulline 10. OPA-Arginine 11. OPA-Alanine 12. OPA-GABA(4-aminobutanoic acid) 13. OPA-Tyrosine 14. OPA-Cys-Cys 15. OPA-Valine 16. OPA-Methionine 17. OPA-Tryptophan 18. OPA-Phenylalanine 19. OPA-Isoleucine 20. OPA-Leucine 21. OPA-Lysine 22. Fmoc-Proline
Column: Inertsil ODS-4 HP 150x3.0mm 3µm (5020-14005)
Mobile phase A: 45/40/15 acetonitrile/methanol/water
Mobile phase B: 20 mM K2HPO4 (pH 6.9)
Gradient:
Time / min | %A | %B |
0 | 11 | 89 |
3 | 11 | 89 |
12 | 22 | 78 |
14 | 28 | 72 |
23 | 30 | 70 |
27 | 65 | 35 |
34 | 75 | 25 |
35 | 100 | 0 |
Flow rate: 0.7 mL/min
Temperature: 35 °C
Detection: Fluorescence excitation 350nm emission 450nm (0-29 min)
Excitation 266nm Emission 305nm (29-35 min)
Injection volume: 1 µL
HPLC - HILIC:
HPLC - Reversed Phase:
HPLC - Reversed Phase:
UHPLC - Reversed Phase:
HPLC - IEX:
HPLC - Reversed Phase:
UHPLC - Reversed Phase:
HPLC - HILIC:
GC:
HPLC - Mixed-Mode:
HPLC - Reversed Phase:
HPLC - Reversed Phase:
HPLC - HILIC:
HPLC - Ion chromatography:
HPLC - Reversed Phase:
UHPLC - Reversed Phase:
HPLC - HILIC:
HPLC - IEX:
GC:
HPLC - Reversed Phase:
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