Vocabulary of analytical chemistry

Lecture 2
Vocabulary of analytical chemistry


Contents
 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

Absolute error:

E = xi - xt (where xt = true or accepted value)
x −x
Relative error:
t × 100%
E = i
r
x
t

 Precision (độ chụm): the agreement between replicate
experimental analyses.

d i = xi − x

individual value

Mean value

Or standard deviation (Sd)= Σd2/n
*Độ chính xác(accuracy) = độ đúng (trueness) + độ chụm
(precission)


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)

Who’s precision better?
Who’s accuracy better?



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 =
kana or Sa = kaCa)
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: 10-4M
 ICP-MS: LOD: 10-9M


Criteria to select an analytical method
 Specificity (độ đặc thù): if the signal depends only on the
analyte.
 Selectivity (độ chọn lọc): a measure of a method’s freedom from
interferences.
 Selectivity coefficient: ka,I= kI/ka
- 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

 Method blank (thí nghiệm trắng): compensate for
interferences, measure the signal of solution that does
not contain the sample
 Calibration curve (đường chuẩn): determine ka using one
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).