Vocabulary of analytical chemistry

Measurements, determinations and analysisAnalyse Phân tích Determine Xác định Measure Đo Analyse drinking water in Hanoi: Determine identities, properties, concentrations of toxic ch

Lecture 2 Vocabulary of analytical chemistry

 Measurements, analysis, determination

 4 levels of methodology

 Criteria to select an analytical method

 Develop a procedure

Measurements, determinations and analysis

Analyse (Phân tích)

Determine (Xác định)

Measure (Đo)

Analyse drinking water in Hanoi:

Determine identities, properties, concentrations of toxic chemicals such as As

Measure absorbance of AsH3 by HVG-AAS

technique

4 levels of analytical methodology

 Technique (kĩ thuật) : any chemical or physical principle to

study an analyte

Exp: AAS; F-AAS; ICP-MS techniques.

 Method (phương pháp): application of a technique for a

specific analyte in a specific matrix.

Exp: F-AAS method to determine Pb in drinking water;

F-AAS method to determine Pb in ores.

 Procedure (qui trình): a set of written directions telling us how

to apply a method to a particular sample, including

information on obtaining samples, handling interferents, and validating results.

A method may have several procedures.

 Protocol (thủ tục) : a set of stringent guidelines specifying a procedure that must be followed if an agency is to accept the results.

APHA: American Public Health Association ASTM: American Society for Testing Materials

EPA: Environmental Protection Agency

Criteria to select an analytical method

 Accuracy (độ chính xác) : agreement between experimental data and the “true” or expected result

 Precision (độ chụm): the agreement between replicate

experimental analyses.

Or standard deviation (Sd)= Σd 2 /n

*Độ chính xác(accuracy) = độ đúng (trueness) + độ chụm

(precission)

Absolute error: E = xi - xt (where xt = true or accepted value)

Er = xi xt xt − × 100%

Relative error:

di = xi − x

individual value Mean value

 Example 1:

 Glucose concentration in blood: 6 mg/L.

 You determine concentration: 5 mg/L

 What’s the error =?

 Example 2:

 You measure 5 times: 5.0; 5.2; 4.7; 5.1; 4.9 (mg/L)

 Your colleague measures 5 times: 4.0; 6.7; 6.8; 6.3; 5.4 (mg/L)



Illustrating the difference between “ accuracy ” and “ precision ”

Low accuracy, low precision Low accuracy, high precision

High accuracy, low precision High accuracy, high precision

Sensitivity (độ nhạy) : Ability to detect the change of signal when there is a change in the amount of analyte (equivalent to ka in S a =

k a n a or S a = k a C a )

Sensitive to change in temperature, hot or cold, skin, hair,

thermometer,

skin: 0.1°, thermometer: 0.1° – 0.001°)

Detection limit (gioi han phat hien) : smallest amount of analyte that can be determined with confidence

Titration: LOD: 10 -4 M

ICP-MS: LOD: 10 -9 M

Criteria to select an analytical method

analyte

interferences

- K can be positive (interferent increases signal),

negative (interferent decreases signal),

magnitude can be > 1 or < 1;

the smaller K, the more selectivity the method

Criteria to select a method

 Robustness (tính ổn định): ability to use an analytical method

on many analytes in a wide variety of sample matrices.

 Ruggedness (tính không ổn định) : in contrast with robustness, ability to provide accurate results despite variations in

executing a method (variation in pH, T, …)

 Scale of operation : the amount of sample available for the

analysis, the expected concentration of analyte in the samples, and the minimum amount of analyte that produces a

measurable signal

Develop a procedure

interferences, measure the signal of solution that does not contain the sample

or more standards.

Develop a procedure

 Sampling (lấy mẫu) : collecting and preparing

samples

(more in Chapter 7)

 Validation (xác nhận giá trị sử dụng của phương

pháp): uses a standard sample, whose composition closely matches the samples that will be analyzed or compared, with a method with known accuracy

(more in Chapter 14).