Sample Analysis Design
Solution Mode
Sample Analysis Design –
Step I – Sample preparation
• The quality of your data will only be as good as the
‘quality’ of your sample –
– i.e. did you adequately prepare your sample in the clean lab?
– With respect to the destruction of matrices for samples requiring
digestion
– Did you adequately spike samples with the correct internal
standard?
– Sample handling protocol is extremely important, e.g. weighing
Sample Analysis Design –
Solid Samples
• Analyze in solid state via LA-ICP-MS?
• Analyze in solid state via SIMS – secondary ion
mass spectrometry?
• Convert into a glass bead and analyze via XRF
– x-ray fluoresence?
• Take powder and digest into solution with acids?
Sample Analysis DesignLiquid (aqueous) samples
– run as-is?
– filter then run?
– dilute then run?
– acidify, dilute, then run?
Sample Analysis Design
• Method of sample preparation also depends upon the
elements of interest
• e.g. don’t analyze your samples in a hydrofluoric acid
medium if you wish to measure Si abundances – why?
• Elemental concentration determinations at ultra-trace
level (ppb, ppt) are very susceptible to contamination
during sample preparation and therefore should be
conducted in clean laboratory environments
Sample Analysis Design –
Clean room environment
• Laboratory ‘clean’ room is a facility in which the
concentration of airborne particles is controlled
to specified limits.
• Eliminating sub-micron airborne contamination is
a control-driven process since contaminants are
generated by people, process, facilities and
equipment. Hence, sub-micron particles must be
continually removed from the air.
Sample Analysis Design –
Clean room environment
• Typical office building air contains from 500,000
to 1,000,000 particles (0.5 microns or larger)
per cubic foot of air.
• A Class 100 cleanroom is designed to never
allow more than 100 particles (0.5 microns or
larger) per cubic foot of air. Class 1000 and
Class 10,000 cleanrooms are designed to limit
particles to 1000 and 10,000, respectively
Sample Analysis Design –
Clean room environment
• A human hair is about 75-100 microns in
diameter. A particle 200 times smaller (0.5
micron) than the human hair can cause
major disaster in a clean room.
• Human hair typically concentrates
elements/contaminants such as Pb!
Sample Analysis Design Sources of contamination
• Facilities:
– Walls, floors and ceilings
– Paint and coatings
– Construction material (sheet rock, saw dust
etc.)
– Air conditioning debris
– Room air and vapors
– Spills and leaks
Sample Analysis Design Sources of contamination
• People:
– Skin flakes and oil
– Cosmetics and perfume
– Spittle
– Clothing debris (lint, fibers etc.)
– Hair
Sample Analysis Design Contamination Control
• HEPA (High Efficiency Particulate Air Filter) Extremely important for reducing contamination filter particles as small as 0.3 microns with a
99.97% minimum particle-collective efficiency.
• Clean room design – requires air flow
dynamics to be the least disruptive as possible –
laminar flow
• Cleaning!
Sample Analysis Design Contamination Control
• Purity of reagents, acids, cleanliness of digestion
vessels, sample bottles, etc can dramatically effect
background levels and data quality
• If possible, use highest purity commercial acids
• At the minimum - sometimes need to further process
reagents – e.g. acid distillation in our laboratory
• Digestion vessels made of disposable fluorinated
polymers (teflon, PFTE, PFA, etc)
• Solutions stored in polypropylene or equivalent
Sample Analysis Design –
Concentration terminology
• Concentrations are typically expressed as either
µg/g, or µg/ml (1 µg - microgram = 1 x 10-6
grams), or ppm, ppb, ppt
– ppm = parts per million = 1 x 10-6 g/g = 1 µg/g
– (µg = microgram)
– ppb = parts per billion = 1 x 10-9 g/g = 1 ng/g
– (ng = nanogram)
– ppt = parts per trillion = 1 x 10-12 g/g = 1 pg/g
– (pg = picogram)
Sample Analysis Design –
Concentration terminology
• E.g. Zircon – ZrSiO4
– ZrO2 = 67.2 wt%
– SiO2 = 32.8 wt%
– Atomic mass of Zr= 91.224
– Atomic mass of Si= 28.0855
– Atomic mass of O= 15.9994
– % Zr in ZrO2 = 91.224/(91.224 + (15.9994*2)) = 74
– % Si in SiO2 = 28.0855/(28.0855 + (15.9994*2)) = 46.7
Sample Analysis Design –
Concentration terminology
• If 100% = 1,000,000 ppm, then
• 74% Zr (out of 67.2 wt%) = 49.73% or 497,300 ppm (or
µg/g)
• 46.7% Si (out of 32.8 wt%) = 15.32% or 153,200 ppm (or
µg/g)
• If you are asked to weigh out 0.00001 g of zircon, then
the amounts of total Zr and Si you would have are:
– Zr = 497,300 µg/g x 0.00001 g = 4.97 µg
– Si = 153,200 µg/g x 0.00001 g = 1.53 µg
Sample Analysis Design –
Concentration terminology
• However, you are asked to prepare a solution of
this zircon sample for ICP-MS analysis in
solution mode;
• then what would be the minimum dilution
factor required given that the maximal amount
of ion signal intensity allowed is 50 x 106 cps
and the yield for both elements in medium
resolution mode is ~60,000 cps/ppb?
Sample Analysis Design –
Concentration terminology
• Maximum allowable concentration is –
• = Max. count rate/ yield = 50 x 106 cps/ 60,000
cps/ppb
• = 833 ppb (ng/g)
• 4.97 µg of Zr needs to be diluted into ? ml of 5%
HNO3.
– 4.97 µg = 4970 ng/833 ng/g = ~5.97 g (ml) of 5%
HNO3
Sample Analysis Design –
Matrix
• Requires a medium (i.e. acid) that
– Provides efficient ion transmission (HNO3 > HCl > H2O)
– Won’t dissolve the glassware of your introduction system – for
example, hydrofluoric acid (HF) would!
– Is relatively cheap and not deemed extremely hazardous to use
– Thus, dilute (1 to 5% volumetric) nitric acid (HNO3) is the acid
medium of choice for most ICP-MS analyses
Sample Analysis Design –
Step 2 – Calibration/Standard Preparation
• Choice of calibration method dependent
upon several factors:
• 1. potential matrix effects
• 2. number of samples
• 3. consistency of matrix across samples
Sample Analysis Design –
Step 2 – Calibration/Standard Preparation
• EXTERNAL CALIBRATION:
• Prepare a set of standard solutions to cover the
expected range of analyte concentrations
• Fit a least squares regression line
• y = mx + b
and calculate analyte concentration in unknowns
Sample Analysis Design –
Step 2 – Calibration/Standard Preparation
23
Na calib curve (Medium resolution)
1000000
900000
800000
700000
y = 17557x
2
R = 0.9992
cps
600000
500000
400000
300000
200000
100000
0
0.00
10.00
20.00
30.00
conc ppb
40.00
50.00
60.00
Sample Analysis Design –
Step 2 – Calibration/Standard Preparation
44
Ca calib curve (Medium resolution)
40000
35000
30000
y = 676.92x
2
R = 0.9961
cps
25000
20000
15000
10000
5000
0
0.00
10.00
20.00
30.00
conc ppb
40.00
50.00
60.00
Sample Analysis Design –
Step 2 – Calibration/Standard Preparation
• Advantages of External Calibration
– Easy to prepare
– Quick
– Widely used technique
Sample Analysis Design –
Step 2 – Calibration/Standard Preparation
• Disadvantages of External Calibration:
• Need to matrix match calibration solutions and samples
• If standards containing <2000 ug/ml (ppm) are being
used, then preparing the standards as simple aqueous
solutions using the acid matrix (5% HNO3) employed for
the samples is sufficient
• HOWEVER, if the samples contain a very high
concentration of one (or more) elements, then this may
not be adequate
Sample Analysis Design –
Step 2 – Calibration/Standard Preparation
• Preparation of External Calibration Solutions:
• Need to evenly space calibration concentrations
• If the highest concentration is much higher than the rest,
linear regression introduces bias favoring the high point
• X = independent variable = concentration
• Y = dependent variable = counts/second
Sample Analysis Design –
Standard Addition Method
• Aliquots of spike are added to unknown samples
to increase the ion signal intensities for elements
of interest
• Typically use at least three aliquots of sample
spiked with evenly spaced amounts of analyte
• These spiked aliquots of sample are used to
generate a calibration line and calculate the
concentration in the sample
Sample Analysis Design –
Standard Addition Method
• S0 = unspiked sample
• S1 = sample spiked with analyte at concentration x
• S2 = sample spiked with analyte at concentration 2x
• S3 = sample spiked with analyte at concentration 3x
• S4 = so on and so on
Sample Analysis Design –
Standard Addition Method
AMT
500000
450000
400000
y = 29387x + 279235
2
R = 0.9992
350000
Cps
300000
250000
200000
150000
100000
50000
0
0
1
2
3
4
Concentration (ppb)
5
6
7
Sample Analysis Design –
Standard Addition Method
• The concentration of the unknown solution is
then determined by dividing the y-intercept value
by the slope of the sample-spike mixing line.
– From example on previous slide,
– Conc. sol’n = 279235 / 29387 = 9.5 ppb
– If the original sample was a solution, then this is the
concentration of the analyte in question in the solution
Sample Analysis Design –
Standard Addition Method
• If the original sample was in solid form that
you digested and subsequently converted
into a solution;
• then in order to determine the
concentration of the analyte in question,
you must factor in the amount of total
analyte in the solution and the dry weight
of the sample powder
Sample Analysis Design –
Standard Addition Method
•
If we continue with the same example, the solution has a
concentration of 9.5 ppb, and the original volume of the unknown
solution was 10 ml (g) prior to aspirating some of it into the plasma
for analysis, then the total amount analyte in the solution is:
•
•
•
= 10 g x 9.5 ng/g (ppb)
= 95 ng, or
= 0.095 µg
•
If the amount of powder weighed out was 0.1 g, then the
concentration of the element in question is:
•
Conc. = 0.095 µg/0.1 g
–
= 0.95 µg/g or ppm
Sample Analysis Design –
Standard Addition Method
• This method works best if the slope of the
calibration line is not too shallow
– This will create more uncertainty in the
location of the intersection between the cps of
your unknown and the calibration line
Sample Analysis Design –
Standard Addition Method
• For maximum precision it’s necessary that the
amount of sample be the same in each aliquot
• Also want the amount of spike added to be the
same for each aliquot
• Amount of spike added should be as small as
possible (usually 0.1 ml to 10 ml total volume)
Sample Analysis Design –
Standard Addition Method
• Ideally, the highest spike concentration
should be approximately equal to the
concentration of analyte in the unknown
• Need to have some idea of the
concentration in the sample prior to
analysis
Sample Analysis Design –
Standard Addition Method
• Advantages:
– Overcomes matrix differences
– More precise and accurate than external calibration
• Disadvantages:
– Requires at least three aliquots for each sample
– Run lengths become much longer and more
preparation time is required