Introduction to Instrumental Methods

Question 1

Differentiate chemical and instrumental method

Answer 1

Chemical method (absolute method)
- analyte concentration is
calculated directly from measurement of the sample.
No additional measurements are
required (other than a measurement
of sample mass or volume).

Instrumental method (relative method)
- The measurement of the sample is compared to measurements of additional samples that are prepared with the use of analyte standards (e.g., solutions of known analyte concentration)

Question 2

Advantages of instrumental methods (5)

Answer 2

1. selectivity
2. speed
3. sensitivity
4. reproducibility
5. small sample requirement

Question 3

Disadvantages of instrumental methods

Answer 3

1. cost
2. complexity
3. maintenance

Question 4

Devices that measure a physical or chemical property of the assayed substance or that measure some factor that enables determination of a property of the substance

Answer 4

analytical instruments

Question 5

General components of analytical instruments (6)

Answer 5

1. signal generator
2. detectors
3. transducers
4. sensors
5. signal processing unit
6. read-out device

Question 6

analytical input that generates electronic signals which results in the production of some form of energy

Answer 6

signal generator

Question 7

amplifies the weak transducer output and is filtered and modified to a form that is acceptable by the output

Answer 7

signal processing unit

Question 8

converts the electrical signal into a form that is usable by the analyst

Answer 8

read-out device

Question 9

mechanical, electrical, or chemical device that identifies, records, or indicates a change in one of the variables in its environment

Answer 9

detectors

Question 10

devices that convert information in nonelectrical domains to information in electrical domains and the converse

Answer 10

transducers

Question 11

class of analytical devices that are capable of monitoring specific chemical species continuously and reversibly

Answer 11

sensors

Question 12

How to consider a detector or instrument as sensitive?

Answer 12

responds to only a small change in analyte concentration

Question 13

Define selectivity

Answer 13

ratio of the sensitivity of an instrument to an analyte to that of an interferant

Question 14

Define detection limit

Answer 14

minimum concentration that can be “detected”, or distinguishes confidently from a blank

Question 15

Define Limit of Quantitation (LOQ)

Answer 15

The minimum concentration of analyte for which an accurate determination of concentration can be made.

Question 16

Define Limit of Linearity (LOL)

Answer 16

The largest concentration for which a calibration curve remains linear

Question 17

Define Linear Dynamic Range (LDR)

Answer 17

The range of concentrations (or signal strengths) between the LOQ and the LOL

Question 18

Most of the time, the response of the instrument is equal to ___

Answer 18

constant x concentration

Question 19

To determine the proportionality constant, the instrument should be ______

Answer 19

calibrated

Question 20

Define calibration and how it is usually determined?

Answer 20

Calibration determines the relationship between the analytical response and the analyte concentration

Usually determined by the use of chemical standards prepared from purified reagents

Question 21

Calibration method where a series of standard solutions is prepared separately from the sample

Answer 21

external standard calibration

Question 22

Describe the calibration curve of external standard calibration

Answer 22

plot of instrument response (y-axis) vs known analyte concentration (x-axis).

A linear signal-concentration relationship will have y = mx + b regression model.

Question 23

Common errors in measurement external standard calibration

Answer 23

1. Not measuring a blank solution (either solvent or reagent blank)
2. Analyzing real samples of a similar matrix but with known analyte concentration
3. No extrapolation of data (working range only)
4. at least 5 concentrations to calibrate

Question 24

Unavoidable experimental errors in the measurement of the intensity values

Answer 24

Uncertainty

Question 25

Define the matrix

Answer 25

The solution, including solvent(s) and all other solutes in which an analyte is dissolved or mixed

Question 26

Describe the matrix effect. How to cancel out its effectd?

Answer 26

refers to the case where the instrumental sensitivity is different for the sample and standards because of differences in the matrix.

Use standard addition calibration method

Question 27

Describe standard addition

Answer 27

1. spiking the sample by adding known amount of a standard solution of analyte to the sample.
2. the responses before and after the addition are measured and used to obtain the analyte concentration

Question 28

Steps in multiple addition methos of standard addition

Answer 28

1. Add same volume of sample
2. Add incremental volume of standard
3. dilute to volume

Question 29

Define internal standard

Answer 29

a reference species (chemically and physically similar to the analyte) that is added to samples, standards, and blanks

Question 30

Describe the calibration curve of internal standard method

Answer 30

ratio of responses (y-axis) vs analyte concentration (x-axis)

Question 31

How to prepare linear regression curve by internal standard method

Answer 31

same amount of internal standard with known/incremental amounts of analyte will be added to the samples of unknown analyte concentration

the ratio of analyte signal/internal standard signal (y-value in y=mx + b) and concentration of analyte will be calculated by solving x.