Www.spectro.com Minimization of matrix effects with environmental samples using ICP-OES.
45
www.spectro.com Minimization of matrix effects with environmental samples using ICP-OES
-
Upload
wilfred-brown -
Category
Documents
-
view
220 -
download
0
Transcript of Www.spectro.com Minimization of matrix effects with environmental samples using ICP-OES.
www.spectro.com
Minimization of matrix effectswith environmental samples
using ICP-OES
www.spectro.com
Agenda
• Introduction
• Matrix effects with environmental applications– Instrumental and analytical approach to avoid/reduce those effects
• Hardware, Optic and introduction systems
• Summary
• Conclusion
2
www.spectro.com 3
CONSISTENCY IS KEY!
Know your HARDWARE!
Know your INTRODUCTION SYSTEM SETUP!!
Know the WORST CASE SAMPLE SENARIO!!!
Only then can you OPTIMIZE your methodology….
www.spectro.com
Concerns;
• Internal Standard usage• Background Correction Points / Models• Atypical IEC’s (Carbon?)• Sample diversity (drinking waters are not waste waters or brines,
etc.)• SPEED!• Getting the best data WHILE trying to follow a regulated
methodology (EPA)…problems of second and third source standards
4
www.spectro.com
Environmental Applications – Challenges
• Emission spectra of environmental samples is typically not extremely line rich and thus environmental work is often described as being “easy”
But
• The matrix composition can vary considerably• The amount of TDS in the final solution can be high vary strongly:
Ca – 3000 mg/l Mg – 500 mg/l
Al – 1000 mg/l Fe – 1000 mg/l
Na – 100 mg/l K – 100 mg/l
P – 1000 mg/l
5
www.spectro.com
Axial Plasma Observation – The choice for environmental work
• Axial plasma observation was introduced in the mid 1990s.
• The development driven by requirements for higher sensitivity, particularly for the toxic, heavy metals.
• Today, axial plasma observation still plays an important role as it provides much higher matrix tolerance as compared to ICP-MS.
6
www.spectro.com
Axial Plasma Observation – Challenges
• Axial plasma observation improves the sensitivity due to the larger observation volume by sampling the emitted light from the entire excitation channel
• However, it also suffers from stronger influences, since all phenomena present in the excitation zone including the plasma tail above the plasma, are viewed
• The most relevant effects are:– Recombination effect– Influence of the plasma load– Self absorption– Easy ionizable element effect
7
www.spectro.com
Recombination effect
• In the cooler temperature regions, within the plasma tail, ions and electrons recombine.
The energy required for their ionization and any excess energy the electrons carry is then again released, which produces additional continuum-like radiation.
8
www.spectro.com
Removal/ minimization the recombination effect
• “Removal” of the recombination zone from the optical path
– The OPI to minimizes the effects from the recombination zone by radial
deflection from the light path
Optical light pathto spectrometer
Adjustableargon jet
Recombinationmatrix effects
Plasma
www.spectro.com
Removal/ minimization the recombination effect
10
Radial plasma viewing completely
eliminates the effect
www.spectro.com
Influence of the plasma load
– With higher plasma loads, the excitation efficiency decreases , which results in lower background to peak rations.
With varying matrices the achievable sensitivity greatly varies. At 1100 W and 10 L/min Ar, the sensitivity difference between a water- and
a 1 % Ca matrix is more than a factor of two
11
1000 1100 1200 1300 1400 1500 1600 1700 1800100
200
300
400
500
600
700
800
1.001.201.401.601.802.002.202.402.602.80
BEC versus Plasma Power - As 189.042 nm
0% Ca
1% Ca
BEC Ratio
Plasma Power [W]
BE
C [
µg/l
]
BE
C-R
atio
www.spectro.com
Optimization of the Plasma Power
– Operating of the plasma with robust plasma conditions and appropriate line selection the effect can be minimized
Using robust conditions at 1400 W and 13L/min and As at 193 nm (less influenced by recombination) the sensitivity difference can be reduced to a factor of 1.18
With radial plasma observation the effect is further reduced
12
1000 1100 1200 1300 1400 1500 1600 1700 1800100
200
300
400
500
600
700
800
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.70
BEC versus Plasma Power - As 193.759 nm
0% Ca
1% Ca
BEC Ratio
Plasma Power [W]
BE
C [
µg/l
]
BE
C-R
atio
www.spectro.com
Self absorption
– Self absorption is the effect where cooler atoms and ions absorb the radiation, they would, under higher temperature conditions, emit
– The effect, reduces the measured light intensity the more of the respective element is present in the sample
Calibration functions become non linear or even reverse, (self reversal of emission lines)
15
www.spectro.com
Elimination of self absorption effects
• High dynamic range detector readout 0.1 m sec phase interval to utilize the full potential of the emission line
Does not influence the self absorption behavior, but enables maximum concentration coverage with a minimum number of lines
• Multi line calibration Selection of appropriate lines providing a linear calibration within the
relevant concentration range Automatic, concentration dependent line switching
• Radial plasma observation Less effected by self absorption Further expansion of the linear dynamic range
16
www.spectro.com
Multi line calibration - Na @ 589.592 nm and 330.237 using Axial View
18
R2 = 0.9995R2 = 0.9999
www.spectro.com 19
Multi line calibration - Na @ 589.592 nm and 330.237 using Radial View
R2 = 0.9997R2 = 0.9999
www.spectro.com
Easy ionizable element effect
– Group 1 and 2 elements cause the strongest effect. Those elements are almost exclusively present as ions
– If introduced at “higher” concentrations, this leads to a massive increase of the electron density and thus a shift of the equilibrium
– It often leads to an enhanced emission for atomic lines and respectively higher than normal intensities for the alkali and earth alkali elements
20
Effect of 50 ppm Na on0.2 ppm K using axial plasma observation
www.spectro.com
Excellent resolution
21
Higher accuracy in line rich matrices
Al 168 nm
Cd 214 nm
8 pm Resolution 23 pm Resolution
www.spectro.com
Ionization buffering and Internal standardization
– The EIE effect can be reduced by providing an excess of easy ionizable
elements in the plasma
– Matrix differences are additionally compensated by the use of an
internal standard• On-line introduction, Sc or Y used as internal standard, Cs used as
ionization bufferPeristalticPump ICP
To nebulizer
From spray chamber drain
Waste
Sample
Buffer/InternalStandard
“orange-orange” peristaltic tubing
“orange-green” peristaltic tubing
www.spectro.com
Calibration of Alkali and Earth Alkali Elements
Calibration StandardsElem.
Std.1[mg/L]
Std.2[mg/L]
Std.3[mg/L]
Std.4[mg/L]
Std.5[mg/L]
Std.6[mg/L]
Na 5000 2500 500 100 10 0K 2500 500 100 10 0 5000Mg 500 100 10 0 5000 2500Ca 100 10 0 5000 2500 500Al 10 0 5000 2500 500 100Fe 0 5000 2500 500 100 10Cs 2000 2000 2000 2000 2000 2000Sc 10 10 10 10 10 10
Elem.
CS7[mg/L]
CS8[mg/L]
CS9[mg/L]
CS10[mg/L]
Na 50 100 3.28 2.34K 1 1 50 100Mg 4000 2000 1000 750Ca 750 4000 2000 1000Al 1000 750 4000 2000Fe 2000 1000 750 4000Cs 2000 2000 2000 2000Sc 10 10 10 10
Axial RadialPower 1450 W 1450 WCoolant flow 12 L/min 12 L/minAuxiliary flow 0.8 L/min 0.8 L/minNebulizer flow 0.95 L/min 0.95 L/minPlasma Torch Quartz, fixed, 2.5 mm
Injector tubeQuartz, fixed, 1.8 mm Injector tube
Spray Chamber Cyclonic CyclonicNebulizer Modified Lichte Modified LichteSample aspiration rate 2.0 mL/min 2.0 mL/minReplicate read time 50 sec per replicate 50 sec per replicate
Plasma Parameters
Check Standards
www.spectro.com
Recoveries of Alkali and Earth Alkali elements usingionization buffering and internal standardization
Sample Al Ca Fe K Mg Na [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l]
Meas. CS7 928 757 2004 1.15 4020 51CS8 721 3934 976 1.06 1985 98CS9 4157 2157 799 51 984 3.65CS10 1943 982 4036 104 758 2.29 Given CS7 1000 750 2000 1 4000 50CS8 750 4000 1000 1 2000 100CS9 4000 2000 750 50 1000 3.28CS10 2000 1000 4000 100 750 2.34 Recov. [%] [%] [%] [%] [%] [%]
CS7 93% 101% 100% 115% 101% 103%CS8 96% 98% 98% 106% 99% 98%CS9 104% 108% 106% 102% 98% 111%CS10 97% 98% 101% 104% 101% 98%
24
Sample Al Ca Fe K Mg Na
[mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l]Meas CS7 990 741 1938 1.11 3852 47CS8 758 3891 966 1.09 1986 97CS9 3982 2047 730 49 985 3.25CS10 2007 983 3816 97 733 2.19 Given CS7 1000 750 2000 1 4000 50CS8 750 4000 1000 1 2000 100CS9 4000 2000 750 50 1000 3.28CS10 2000 1000 4000 100 750 2.34 Recov. [%] [%] [%] [%] [%] [%]
CS7 99% 99% 97% 111% 96% 93%CS8 101% 97% 97% 109% 99% 97%CS9 100% 102% 97% 98% 98% 99%CS10 100% 98% 95% 97% 98% 94%
Axial View Radial View
Slightly better average recoveries are achieved in radial mode (99% versus 102%)
www.spectro.com
Influence of an Alkali Matrix (50 ppm Na )on Alkali Elements (K) using Axial and Radial Plasma Observation
25
Axial: Strong Effect
Radial: Effect visible,but greatly reduced
Axial
Radial
Depth of field
Viewing volume
Induction areaInduction coil
Viewing volume
Viewing height
Central channel
Induction coil
www.spectro.com
Twin Interface Plasma Observation
• Accurate determination of alkali elements in the presence of a varying alkali/earth alkali matrix (e.g. mineral waters) with axial ICP-OES
The EIE (Easy Ionizable Element) effect is greatly reduced since the alkali/earth alkali elements are measured in radial mode
No need for the use of an ionization buffer Cost reduction, reduced risk of contamination
Toxic elements can still be determined with great sensitivity since they are measured in axial mode
Dynamic range and linearity can be further expanded
www.spectro.com
SPECTROBLUE Twin Interface - Principle
28
Coupling mirror Optic window
Control mirror
Mirror, plain Mirror, concave
Torch
Entrance slit optic
Radial view
Axial view
www.spectro.com
Twin Interface - Design
30
• Small, light throughput optimized light path
• Periscope optic rigidly connected to the optic and the torch.
• Plasma sided, easy to clean/change, window to avoid the contamination of optical components
• Self adjusting, pneumatic control mirror
• All components easily exchangeable
www.spectro.com
New OPI Flange and Locking Mechanism
The OPI can be disassembled without removal of the torch
Novel, easy to use locking mechanism
www.spectro.com
Twin Interface - Properties
• Switchable 3-Mirror Periscope• Optimized for smallest size and maximum light throughput using
“Reverse Ray Tracing”• Self adjusting, pneumatic control mirror• Robust construction. Rigidly coupled to the optical system and the
plasma torch• 45°- viewing angle to reduce contamination• Plasma sided protection windows to avoid contamination of optical
components• All components easy to maintain• Separate, computer controlled light path purge flows
32
www.spectro.com
Axial Plasma Observation – The choice for trace analytical work
• Axial plasma observation was introduced in the mid 1990s
• The development driven by requirements for higher sensitivity, particularly for the toxic, heavy metals
• Axial plasma observation still plays an important role as it provides much higher matrix tolerance as compared to ICP-MS
33
www.spectro.com
Axial Plasma Observation – Challenges
• Axial plasma observation improves the sensitivity by sampling the emitted light from the entire excitation channel
• However, it also suffers from stronger influences, since all phenomena present in the excitation are viewed
• Pros and Cons:– High sensitivity– Stronger matrix effects– Less suitable for high TDS and
applications with organic solutions
34
Viewing volume
www.spectro.com
Radial Plasma Observation – The choice for high stability, high matrix loads and organic solutions
• Radial plasma observation provides lower sensitivity since the central channel is only partially view
• However, it provides high stability and freedom from matrix effects since the affected zones in the plasma are blanked out
• Pros and Cons:– High stability– High matrix tolerance– High linear dynamic range – Freedom from matrix effects– Lower sensitivity
Viewing volume
www.spectro.com
“Dual View” – really the best of both worlds?
• “Dual View” appears to solve the dilemma having to make a choice between axial and radial plasma observation
But!• Only one view uses the direct light path, the other is compromised
Sensitivity loss due to additional opticalcomponents
Sensitivity loss in the UV since thelight path cannot be purged effectively
36
Coupling mirror Optic window
Mirror, plain Mirror, concave
Torch
Entrance slit optic
www.spectro.com
“Dual View” – really the best of both worlds?
• For highest sensitivity, instrument using a “Dual View” technique typically have a horizontal torch orientation Disadvantages with higher matrix loads and organic solutions
• Compared to a dedicated radial interface the sampling volumeusing a periscope is smaller Ultra high stability and precision as compared to a dedicated radial
systems cannot be achieved
The “dual view” technique serves a purpose, but does not provide the highest possible performance
37
www.spectro.com
Other hardware options?
Turn the plasma!
• Using MultiView, the plasma orientation can be changed
The instrument can be used as a dedicated radial as well as a dedicated axial instrument
38
www.spectro.com
MultiView - Radial/Axial Plasma in one Instrument
39
Analytical performance without compromise
Highest sensitivity Highest stability and precision Highest dynamic range Highest matrix compatibility Freedom from matrix interferences
Other techniques or instruments are not anymore required
www.spectro.com
TI Analytical Performance: LODs TI axial mode
40
Line LOD Appl. Report
EOP
TVO-LOD Limit
nm ppb ppb ppbAs 189.042 0.79 0.81 1Sn 189.991 0.26 0.28Tl 190.864 0.58 0.55Se 196.09 0.96 0.98 1Sb 206.833 0.88 0.87 1Zn 213.856 0.04 0.05Cd 214.438 0.04 0.04 0.5Pb 220.351 0.63 0.68 1Ni 231.604 0.17 0.19 2Cr 267.716 0.14 0.15 5Cu 324.778 0.28 0.31Na 589.592 0.29 0.4Li 670.784 0.04 0.04K 766.491 1.50 1.7K 769.896 2.10
• Integration time 60s per replicate• TI-Torch with slit
– No visible wear after 200h of operation
• TI/DV in axial mode achieves the exact same sensitivity as a dedicated axial (EOP) system BUT the radial mode is less sensitive and throughput reduced!
www.spectro.com
TI Analytical Performance: Linearity, K 766.491 nm
41
• The linearity is drastically reduced for elements effected by the EIE
• Linearity: R2=0.99• Rel. deviation of the standards up to 25%
• TI Radial: Linearity, R2=0.99998• Rel. deviation of the standards < 5%
Axial mode Radial mode
www.spectro.com
Analytical Performance: Linearity radial mode
42
• Linearity: R2=0.99994• Rel. deviation of the standards < 7%
www.spectro.com
Summary
• Matrix effects with environmental applications can effectively addressed– Recombination effects can be minimized by a deflection of the
recombination zone (OPI) or eliminated by radial plasma observation– Robust plasma conditions minimize the differences in excitation
properties in the plasma caused by a varying and higher TDS matrix– Self absorption effects can be avoided by multi line calibration and an
appropriate line selection.• A high dynamic range read out helps to reduce the required number of lines• Self absorption effects are drastically reduces by radial plasma observation
– The EIE effect can be well reduced by the use of an ionization buffer– Twin Interface Plasma Observation eliminates the EIE and further
expands the dynamic range
43
www.spectro.com
Conclusion
• EOP, the axial interface version, provides high sensitivity and detection limits for superior analysis of industrial and environmental trace elements.
• SOP radial interface version offers precise performance at higher sample concentrations, exhibiting excellent tolerance for high saline and organic fractions plus superb analysis of suspensions and slurries.
• TI twin-interface version automatically performs both axial and radial viewing of the plasma. This “eliminates” the EIE effect, optimizes linearity and dynamic range, while enabling high-sensitivity measurement of toxic elements.
• MV, Multi View interface allows the end user to SWITCH between a DEDICATED Radial AND Axial system (without the drawback of a periscope or complicated entrance optics). This offers the best of both worlds in a systems that is optimized for Axial and Radial viewing, something NO DUAL VIEW system can achieve!
44
