The pretreatment of samples is a complex task when analyzing test substances, and it holds significant importance in the overall analysis and detection process. It also plays a crucial role in identifying the source of detection errors. This article aims to summarize four fundamental pretreatment methods used for samples in atomic absorption spectrometry (AAS) analysis. Additionally, it discusses the basic detection methods for six types of samples commonly encountered in routine analysis and detection. These methods are practical and can serve as valuable references for users of AAS instruments.
Wet Digestion Method:
For samples weighing approximately 0.1000 to 0.5000g, the commonly used approach involves utilizing mixed acids. The following acid ratios are often employed:
(1) HNO3:HCLO3 = 5:1
(2) HNO3:H2SO4 = 5:1
(3) HNO3:HCl = 5:1
(4) Pure HNO3
Note: It is crucial to avoid the presence of volatile (acetone, ether, ethanol, etc.), flammable, and explosive substances during the digestion process. The wet digestion method is widely employed and well-known, so further elaboration is unnecessary.
Dry Ashing Method:
Typically, samples weighing between 2.000 and 5.000g are treated using the dry ashing method, which prevents volatilization. The procedure involves placing the sample in a porcelain crucible, adding a few drops of water to wet it, followed by a small amount of concentrated nitric acid. Heat is then applied to carbonize the sample over a low flame. The crucible is subsequently transferred to a muffle furnace for ashing at approximately 550°C for 2 to 4 hours. After cooling, the ash (colorless or light in color) is dissolved using other acids, often in a 1:1 ratio with nitric acid (varies depending on the sample). The solution is then filtered, adjusted to volume, and aliquoted into 10mL, 25mL, and 50mL portions for further use.
High-Pressure Tank Method (Using a Lidded Tank Made of Polytetrafluoroethylene):
When the sample weighs less than 0.3000g, this method is employed. It involves adding 6mL of mixed acid and 1mL of HF(H2O2) to the sample. The autoclave is sealed, and the sample is heated at 160°C for 5 hours. After cooling, the solution is filtered and adjusted to the desired volume for subsequent analysis.
Microwave Digestion Method:
The microwave digestion method utilizes commonly used mixed acids, including:
(1) HNO3:HCLO3
(2) HNO3:H2SO4
(3) Pure HNO3
Note: The choice of specific acid for digestion varies depending on the sample. Readers are encouraged to select the appropriate acid accordingly.
AAS Analysis and Detection Methods for Various Samples:
Analysis of Pb, Cd, As, Mo, Cr, etc. (Graphite Furnace AAS Method):
For Pb analysis, a 1.0mL sample is diluted to 10mL with 1% HNO3. The linear range is 0 to 20ng/mL, with drying temperature set between 80 and 100°C, ashing temperature at 200°C, and atomization temperature at 1500°C.
For Cd analysis, a 1.0mL sample is diluted to 10mL with deionized water. The linear range is 0.1 to 0.4ng/mL, with the same drying, ashing, and atomization temperatures as Pb.
For As analysis, a 1.0mL sample is combined with 100μL of Ni (2mg/mL) and diluted to 10mL with 1% HNO3. The linear range is 0 to 4ng/mL, with the same drying, ashing, and atomization temperatures as Pb.
For Mo analysis, a 1.0mL sample is diluted to 10mL with 1% HNO3. Pd is used as the modifier, and the linear range is 0 to 20ng/mL, with the same drying, ashing, and atomization temperatures as Pb.
For Cr analysis, 1mL of the sample is diluted to 100mL with deionized water. The linear range is 0 to 40ng/mL, with the same drying, ashing, and atomization temperatures as Pb.
Analysis and Testing of Se, K, Na, Ca, Mg, etc. in Plant Samples:
For Se analysis in forage grass, a 1mL sample is combined with 100μL of Ni(NO3)2 (3mg/mL) and diluted to 10mL with a mixture of 1% HNO3 and 0.1% Triton (1:1). The linear range is 0 to 20ng/mL, with the same drying, ashing, and atomization temperatures as Pb.
For K and Na analysis in plants, 0.2000g of the sample is weighed into a polyethylene bottle, and 10mL of mixed acid is added. The extraction is performed in a constant temperature water bath at 90°C for a specific period. After filtration to a volume of 50mL, appropriate aliquots are taken for analysis.
For Ca and Mg analysis in plants, the sample is incinerated at 550°C, dissolved with hydrochloric acid (1+1), and adjusted to a volume of 50mL. A modifier such as 5% CsCl2 (2mL) is added for analysis, as appropriate.
Be Analysis in Mushrooms and Tea (Transverse Heating AAS Method):
The sample is washed, dried, fixed at 105°C, further dried at 70°C, and crushed after cooling. The resulting powder is sieved through a 60-mesh sieve before analysis. For Be determination, 1.0000g of the sample is weighed into a 150mL beaker, and 15mL of HNO3 and 1mL of H2SO4 are added. The beaker is covered with a watch glass, and low-temperature digestion is performed until white smoke appears. After dilution to 25mL, appropriate aliquots are taken for analysis.
The linear range for Be is 0 to 8ng/mL, with drying, ashing, and atomization temperatures set at 130°C, 1500°C, and 2300°C, respectively.
Ge Analysis in Beverages (Using Transversely Heated Flat Graphite Tubes):
The linear range for Ge analysis is 0 to 200ng/mL, using a Ni(NO3)2 improver concentration of 50μg/mL in a 1% HNO3 medium. The instrument conditions during analysis include a drying temperature of 130°C, ashing temperature of 800°C, and atomization temperature of 2000°C.
Ca Detection in Steel Slag (Generally Using Flame AAS Method):
For Ca analysis in steel slag, a sample finer than 200 mesh is taken. A quantity of 0.0500g is weighed into a 50mL PTFE crucible, moistened with a small amount of water, and dissolved using a mixture of water, nitric acid, and hydrofluoric acid in a ratio of 5:8:6. Sulfuric acid (2mL) is added, and heating continues until white smoke appears. The volume is adjusted to 50mL with water for analysis. As steel slag often contains niobium, which interferes with calcium analysis, the addition of improvers such as Triton-100 (20%), Vc (0.1 M/L), and 2% HNO3 is highly effective.
Determination of Metal Elements in Blood Samples:
Human blood contains various trace substances, including inorganic salts, metabolites, O2, hormones, enzymes, antibodies, and trace elements. These substances can be detected by various methods, including AAS, especially for heavy metal elements that can be beneficial, toxic, or harmful to the human body. Specific digestion methods can be found in references “2, 3), (3)”. For example:
Determination of Cu in serum (Flame AAS Method): 0.8mL of serum is diluted to 10mL with 1% HNO3 for analysis.
Determination of As in serum (Graphite Furnace AAS Method): The sample is diluted with 1% HNO3, and Ni(NO3)2 is used as a matrix modifier during analysis.
Determination of Cd in serum (Graphite Furnace AAS Method): 2mL of serum is slowly mixed with 1mL of HNO3 and 0.5mL of H2O2. The mixture is heated on an electric hot plate, and additional HNO3 and H2O2 are added until light yellow. After dissolving with 4mL of deionized water, appropriate aliquots are taken for analysis.
Determination of Fe and Cu in albumin (Flame AAS Method): 1mL of the sample is combined with 7mL of mixed acid, heated until white smoke emerges, and diluted to 25mL. Samples are then taken for analysis as needed.
Determination of germanium in whole blood (Graphite Furnace AAS Method): 1.0mL of venous blood is diluted four times with 0.2% Triton-100, and 20μL of the diluted solution is injected. A mixture of strontium (30μg) and ammonium nitrate (10μg) is used as a matrix modifier to enhance sensitivity. The ashing temperature is set at 1000°C, and the atomization temperature at 2500°C.
In conclusion, the pretreatment of samples is a critical step in the analysis of atomic absorption spectrometry . This article has presented four essential sample pretreatment methods and discussed the detection techniques for various types of samples commonly encountered in routine analysis. AAS users can refer to these methods to guide their analyses effectively.
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