Define Problem

Select Appropriate Methods

Obtain and store sample

Pre-treat sample

Apply necessary statistical methods

Compare results with standards

Perform required measurements

Present results to customer

Present results in a format understandable to analyst

How do you decide the best How do you decide the best method for your problem?method for your problem?

Factors of the method Factors of the method which will influence method which will influence method choicechoice

1. Precision: Provides a measure of random error 1. Precision: Provides a measure of random error in a measurementin a measurement

Precision vs. accuracyPrecision vs. accuracy

Accuracy and PrecisionAccuracy and Precision

Accuracy determined by Accuracy determined by factors including factors including calibration, lab calibration, lab technique, sample technique, sample storage, collection, etc.storage, collection, etc.

Precision limited by Precision limited by noise alonenoise alone

Measured by:Measured by: Std deviationStd deviation Variance = sVariance = s22

Coefficient of variance Coefficient of variance

CV = s/x * 100CV = s/x * 100 Relative std. dev.Relative std. dev.

RSD = s/xRSD = s/x

Factors of the method Factors of the method which will influence method which will influence method choicechoice

2. Bias: systematic error2. Bias: systematic error

How do you know when you have bias?How do you know when you have bias?

Can come from:Can come from:

Instrumental errorsInstrumental errors

Personal errorsPersonal errors

Method errorsMethod errors

3. Sensitivity:3. Sensitivity:Which metal method is most Which metal method is most sensitive?sensitive?

4. Detection limit4. Detection limit

Determines whether or not a signal has been Determines whether or not a signal has been detected and the confidence such a decision detected and the confidence such a decision can be madecan be made

Detection limits and Detection limits and noisenoise

Noise often dictates Noise often dictates how low a detection how low a detection limit will belimit will be

Detection limit at least Detection limit at least 3x std deviation of 3x std deviation of blank can be as high as blank can be as high as 100X100X

Which one has the lower detection limit?

5. Concentration range5. Concentration range

Working range where calibration curve is Working range where calibration curve is linearlinear

6. Selectivity6. Selectivity

Extent to which Extent to which method is free from method is free from interferencesinterferences

Which method is better for drug testing, Mass Spec or TLC?

More about NoiseMore about Noise

1.1. Noise is everywhereNoise is everywhere2.2. Noise is usually constant in magnitudeNoise is usually constant in magnitude3.3. Signal to noise ratio is like GPASignal to noise ratio is like GPA

(figure or merit) See transparency(figure or merit) See transparency

What causes noise?What causes noise?

1. Chemical noise: 1. Chemical noise: Due to undetected Due to undetected (to the scientist) (to the scientist) changes in the changes in the chemistry of the chemistry of the reaction. Temp, reaction. Temp, glassware, fumes, glassware, fumes, etc.etc.

2. Instrumental Noise2. Instrumental Noise

a. Johnson or thermal noise: a. Johnson or thermal noise: Due to movement of Due to movement of electrons or other charge electrons or other charge carriers Also called white carriers Also called white noise b/c occurs at all noise b/c occurs at all frequenciesfrequencies

Where: Where: rmrms = root mean s = root mean

square noisesquare noise

k = Boltzman constantk = Boltzman constant

T = absolute tempT = absolute temp

R = resistanceR = resistance

f = instrument’s bandwidthf = instrument’s bandwidth

How would you minimize?

b. Shot noise (much smaller b. Shot noise (much smaller than Thermal noise)than Thermal noise)

Due to individual Due to individual charge carriers charge carriers moving across n moving across n p p junction or arriving at junction or arriving at electrode surfaceelectrode surface

iiavav = shot noise = shot noise

I = intensity of signalI = intensity of signal

e = charge on ee = charge on e--

f = band widthf = band width

c. Flicker noisec. Flicker noise

Observed at low frequencies (< 100Hz)Observed at low frequencies (< 100Hz)

d. Environmental Noised. Environmental Noise

See transparencySee transparency Most common are electric and magnetic Most common are electric and magnetic

fields @ 60 Hz and harmonics, 120, 180, fields @ 60 Hz and harmonics, 120, 180, 240, etc.240, etc.

Proper grounding of instruments and Proper grounding of instruments and transmission will lower noisetransmission will lower noise

How can you enhance your How can you enhance your signal relative to your signal relative to your noise?noise?

HardwareHardware

1. Low pass filters1. Low pass filters