High-Performance Agronomy Analysis Using ICP-OES · 2020. 7. 8. · physiology, soil fertilization, and soil science — improves the use of soil and increases the production of food

A WHITE PAPER FROMSPECTRO ANALYTICAL INSTRUMENTS

Introduction

Agronomy — including plant genetics, plant

physiology, soil fertilization, and soil science

— improves the use of soil and increases the

production of food and fiber crops.

The work often relies heavily on analysis of

field samples. So the accuracy and timeliness

of these analyses are vital to the testing

regimes of state regulatory laboratories,

contract and other commercial/university

soil laboratories, and production laboratories

maintained by fertilizer makers and other

manufacturers.

Recent technological improvements

have further enhanced the capabilities

of inductively coupled plasma optical

emission spectrometry (ICP-OES) analyzers

for agronomic analysis. This paper will

detail those improvements, and outline the

qualifications an analyzer needs for efficient

analysis of soils, plant materials, and

fertilizers.

When results matter

High-Performance

Agronomy Analysis

Using ICP-OES

2 High-Performance Agronomy Analysis Using ICP-OES

• Advanced detector technology: with

high levels of stability where accuracy

is critical; speed to handle the highest

sample throughputs; and dynamic range

to analyze trace levels as well as high

concentration levels of major elements,

without the need for additional sample

prep.

• Software that offers superior “forensic”

capabilities. Since soil testing is often a

trending analysis, archiving all previous

results is a critical feature.

• Complete freedom in selecting the

method’s internal standard, for optimal

stability.

• Superior accuracy through type

standardization. High accuracy is

especially critical for samples such as

fertilizers and fertilizer by-products.

• Re-work prevention: since the complete

spectrum is stored with every measure-

ment, the analytical results can be post

analytically re-evaluated at any later point

in time, without re-analysis of the sample.

• High-throughput sample analysis

functions such as intelligent rinsing and

moving. These help minimize sample-to-

sample time without the added costs of

cumbersome injection valves.

Why your choice of analyzer matters Agronomical analyses can demand a lot

from an instrument. The ideal spectrometer

design for these tasks delivers exceptional

accuracy, high stability, and high speed,

coupled with low cost of analysis and

minimal downtime.

To achieve these goals, the analyzer should

possess the following characteristics:

• An optical system with high resolution,

stability, and sensitivity. Resolution and

stability are especially critical for analysis

of soils containing line-rich elements;

fertilizer analysis puts a premium on

stability for precision and accuracy; and

sensitivity is key for plant tissue analysis.

• A generator with high stability and

robustness. It also provides the ability to

handle high amounts of total dissolved

solids (TDS) in samples without dilution.

Less dilution means fewer errors, plus

faster sample preparation.

• A dedicated plasma interface — with the

necessary matrix compatibility, sensitivity,

and freedom from interferences to

provide acceptable performance for a

number of challenging elements.

3High-Performance Agronomy Analysis Using ICP-OES

• Multi-client capability. This allows

multitasking teamwork and increased

operational efficiency.

Not all analyzers are created equalUnfortunately, many ICP-OES analyzers on

the market struggle or fail to meet some of

the above criteria.

The trouble with echelle

For instance, a surprising percentage of

ICP-OES analyzers available from major

manufacturers use optical systems based

on echelle technology. (French for “ladder,”

this describes a particular grating pattern.)

When it comes to attempting certain types

of analyses needed in many agronomy labs,

echelle-based systems suffer from a number

of drawbacks:

• Interferences — order overlaps on

particular elements

• Stray light — higher background levels

and lower sensitivity

• Wavelength-dependent resolution

— quite low in the range above 250

nanometers (nm); this is troublesome for

line-rich matrices

• Lower stability — because all echelle-

based systems use purged optical

systems

• Complete inability to analyze halogens at

levels below parts per million (sub-ppm)

Their purged optical systems and

breakdown-prone external chillers also

mean these instruments tend to encounter

more downtime, as well as greater

consumables and lifetime costs.

The advantages of ORCAInstead of problematic echelle-based

systems, agronomy analyzer buyers would

be well advised to consider advanced

ICP-OES instruments based on Optimized

Rowland Circle Alignment (ORCA) optical

technology. This features an optical

arrangement with entrance slit, concave

grating, and detector all positioned on a


circle. The design minimizes the number

of required optical components, optimizes

light throughput, and limits light losses

during diffraction. A prime example: the

SPECTRO ARCOS analyzer.

This type of analyzer can deliver the

characteristics listed above under “Why

your choice of analyzer matters,” including

these optical system advantages:

• Better line separation in line-rich spectra

— Minimization of spectral interferences

— Simplified method development

when dealing with line-rich matrices

— Improved accuracy via fewer

corrections

• Use of a multiplicity of non-interfered

lines

• High light throughput (including in the

vacuum ultraviolet or VUV spectral

range)

• Full sensitivity for all elements

• Wide spectral range — from 130 to 770

nm; including the ensured ability to

analyze halogens such as chlorine (Cl),

iodine (I), and bromine (Br) below 165 nm

• Few optical components

• Low stray light levels — for as much as a

5x increase in sensitivity

• Ensured robustness

To sum up, optical and other shortcomings

may hinder some echelle-based

instruments in certain agronomical

analyses. Example: for plant tissue testing,

(For a more detailed comparison,

see the white paper „Which

Spectrometer Technology Offers

Superior Performance? Echelle vs.

ORCA“ at http://icp-oes.spectro.com/

echelle-vs-orca)


they can struggle or fail in attempting to

analyze low concentration levels of arsenic

(As), selenium (Se), molybdenum (Mo), lead

(Pb), and thallium (Tl) when residual organic

carbon (C) is present in a sample. (See other

examples in the sections below.)

By contrast, with the industry’s best

transparency in the spectral range below

180 nm, truly advanced ICP-OES analyzers

can measure all these elements to the levels

of accuracy the agronomy lab requires.

The importance of plasma viewing

Another important consideration in

analyzer design for many agronomy tasks:

plasma viewing flexibility. Radial (from the

side) and axial (from end to end) plasma

viewing systems offer differing benefits and

drawbacks.

Radial plasma observation:

• Larger linear range

• Higher matrix compatibility

(up to 20% TDS)

• Fewer matrix effects

• Easier handling

• Better stability

Axial plasma observation:

• Lowest detection limits

• Routine matrix compatibility

(up to 10% TDS)

• Higher flexibility

Instruments offering only axial view may be

less expensive, and supply high sensitivity

for trace analysis. But they can also suffer

from drawbacks such as recombination

effects, influences from the plasma load,

self-absorption, and the easily ionizable

element (EIE) effect. These may hinder

certain agronomical analyses. For instance,

EIE is especially prevalent in many soil

samples containing alkali elements such as

sodium (Na) or potassium (K). Users must

contend with chemical buffers; extra effort

and cost; and more risk of contamination. To

eliminate those effects, the recommended


plasma observation technology for soil

analysis is the radial view.

Conventional ICP-OES analyzers usually

come with one view: radial or axial. Some

systems instead have dual-view technology;

however, this involves compromising one

view in favor of the other — privileging axial

or radial observation with a direct view,

while adding an indirect view capability via

a small periscope for the other observation

mode. A truly advanced analyzer (such

as SPECTRO ARCOS MV) instead offers

a unique MultiView option. A simple

mechanical adjustment allows the user to

literally “turn” the instrument from one view

to another, in 90 seconds or less. The result

is true radial view or true axial view, without

any compromise in performance.

The advanced analyzer difference

Besides optical differences, advanced ICP-

OES analyzers such as SPECTRO ARCOS

furnish a number of other advantages.

They possess the latest generator

technology, so they can operate efficiently

even with the highest amounts of TDS

found in many agronomy samples. These

systems also stabilize very quickly — in


less than 10 minutes — to accelerate lab

productivity should the instrument be in less

than constant use.

SPECTRO ARCOS features an air-cooled

design instead of expensive, breakdown-

prone external chillers. Its sealed optics

eliminate the need for constant argon (Ar)

purges or additional gases such as nitrogen

(N2). And its OPI-Air interface eliminates

the need for air compressors/air jets or

for nitrogen generators to remove the

recombination zone from the optical light

path.

Advanced software provides streamlined

workflows, simplified method development,

accelerated data handling, and high

productivity. It also furnishes full forensic

capabilities, including powerful audit trails

covering every event and change, plus rapid

record retrieval — for total traceability.

These are among the design advantages

that give advanced analyzers outstanding

performance; proven reliability; lower

consumables cost; less downtime and

maintenance; and lower lifetime operating

costs.


Analyzers differ, as explained above. The

needs of labs performing various agronomy

analyses may also differ. Below are some

specifics of laboratory requirements for

analyses of soils, plant materials, and

fertilizers.

SoilsLaboratory analysis of soil samples demands

a number of critical analyzer characteristics.

Throughput and speed

The analyzer must be able to process very

high sample quantities. In many cases,

extremely large amounts of data must be

analyzed within tight seasonal time frames.

So soil labs demand maximum sample

throughput to improve productivity and

reduce operating costs. This puts a premium

on the highest possible speed of analysis,

sample-to-sample.

Many conventional ICP-OES instruments

waste precious time performing sequential

measurement: taking each element in a

sample one at a time. However, advanced

ICP-OES analyzers such as SPECTRO ARCOS

deliver fully simultaneous measurements

to maximize speed of analysis. In addition,

features such as an “intelligent move”

system cut sample-to-sample time —

without the extra cost of expensive so-

called “express” valves.

Uptime

Especially in busy seasons, soil labs demand

maximum analyzer uptime. Advanced

analyzers possess features to ensure

continuous operation with low maintenance.

Precision and sensitivity

Soil analysis labs also require superior

measurement capabilities — to levels

of precision and accuracy that many

conventional models can’t supply. Consider

sensitivity. Advanced ORCA-based ICP-OES

models provide simultaneous spectrum

capture in a wide 130-770 nm wavelength

range, with up to 5x more sensitivity

than echelle-based systems. For some

critical elements in the VUV the sensitivity

improvement is even higher.

Alkalis

Most soil samples contain alkalis and earth

alkalis. These challenge many analyzers by

emitting elevated levels of stray light. The

result: loss of sensitivity. This makes trace

measurements difficult for echelle-based

systems. By contrast, when running soil


samples in comparable conditions with

short integration times, an advanced ORCA-

based analyzer such as SPECRO ARCOS can

deliver higher sensitivity and up to twice the

precision of conventional models.

Sulfur

Results are even better when determining

trace levels of sulfur (S). Here, these

advanced analyzers can accurately

determine sulfur to parts per billion (ppb)

levels in solution, with less than 8 seconds’

integration time — and greater sensitivity!

Halogens

Advanced analyzers also provide excellent

results for the sub-ppm measurement of

halogens: especially chlorine and bromine.

Plant materialsLaboratories running plant tissue analyses

— primarily for micronutrient testing —

need analyzers with similar but not identical

capabilities to soil lab models.

Uptime, costs, speed

Naturally, plant materials testing labs also

need analyzers that offer low maintenance

and low operational cost. Advanced ICP-

OES analyzers provide both.

As with soil laboratory users, plant material

analysts feel the need for speed. Fortunately,

SPECTRO ARCOS, for instance, can deliver

sample-to-sample cycles of less than 25

seconds. And again, “intelligent move”

and “intelligent rinse” systems cut sample-

to-sample time in routine cases — without

costly express valves.


Molybdenum

To verify nutrient levels in vegetal materials,

the instrument should be able to not

only detect but quantify trace levels of

molybdenum. Advanced ICP-OES models

such as SPECTRO ARCOS possess a limit

of quantitation (LOQ) for molybdenum (Mo)

down to 2 ppb in solution.

In addition, molybdenum determination

requires a high degree of analyzer flexibility.

Users of SPECTRO ARCOS with Multi-

View capability can run routine analysis

with radial plasma observation, for speed

and higher cost efficiency, as well as

better results for alkalis such as sodium

or potassium. But when a sample requires

research-grade analyses, the user can

switch over to axial view within 90 seconds

— and get molybdenum results down to

2 ppb. Note that conventional dual-view

instruments can offer only compromised

axial performance, and are challenged to

measure molybdenum at these exceptional

LOQs.

Boron

Another key: the ability to measure boron

(B). This is especially critical in research

labs. While many analyzers have difficulties

with elements displaying emission lines

in the lower UV range (they are forced to

identify boron using a spectral line that has

an iron interference — problematic if a given

sample contains both boron and iron (Fe)),

advanced ICP-OES models have no trouble.

Alkalis

The ability to achieve linearity/accuracy

in measuring alkalis is also critical for

plant tissue testing. Users who rely on

conventional axial-view or compromised

dual-view instruments face difficulties here:

such as a need to use matrix modifiers,

overall slow analysis times, and the long-

term effects of chemical attack on torch

materials. Instruments equipped with Multi-

View have no such problems, since they

can handle alkalis using a true radial-view

system.

Chlorine

Plant tissue analyses also require ensured

measurement of chlorine (Cl). This is

critical in determining plant nutrient uptake

rates. Conventional ICP-OES analyzers

simply can’t perform this analysis to sub-

ppm levels with any degree of success.

Fortunately, an advanced ICP-OES analyzer

can, with relative standard deviations

(RSDs) generally below 4%.


FertilizersCritical requirements for spectrometric

analysis of fertilizers are also demanding.

These include the high levels of throughput,

speed, cost efficiency, and productivity

mentioned above.

Phosphorus and potassium

There’s a vital need to measure the nutrients

phosporus (P) and potassium (K) with high

accuracy. However, neither axial nor dual-

view plasma viewing systems can do so.

Accurate determination requires radial

plasma observation; axial systems lack

the required precision and linearity and

are prone to matrix effects, thus impairing

accuracy. Conventional dual-view systems,

with their compromised radial component,

may compensate successfully for EIE

effects, but again fall short for precision and

linearity, and thus accuracy.

Fortunately, an advanced ORCA-based

instrument with optimized radial plasma

observation — such as SPECTRO ARCOS

with MultiView — can provide accurate

measurements for both potassium and

phosporus.

Antimony, lead, and arsenic

Trace impurities in fertilizers and by-

products demand analysis on a radial

system, to avoid problems commonly

encountered when trying to measure alkalis.

But conventional ICP-OES radial models lack

the sensitivity needed to measure heavy

metals in the relevant matrices. Excessive

stray light greatly impairs their ability to

handle antimony (Sb), lead (Pb), or arsenic

(As). An advanced analyzer with dedicated

radial plasma observation and an ORCA

optical system is required instead.

Bromine

Bromine (Br) must also be closely analyzed,

in trace amounts, since it can adversely

affect process catalysts. However, the single-

digit ppm measurements required are not

possible on conventional echelle-based ICP-

OES devices. Similarly, it takes an advanced

ICP-OES analyzer with high sensitivity in the

VUV spectral region to measure the ultra-

low bromine concentrations encountered in

potash mines.

Chlorine

Fertilizer testing also necessitates precise

analysis of chlorine (Cl) — especially in

manufacture of fertilizer by-products,

such as high-purity phosphorus (P) for

semiconductors. Conventional ICP-OES

analyzers simply can’t analyze sub-ppm

chlorine levels with any degree of success.

An ICP-OES analyzer with advanced VUV

capability is a necessity.

Backup

A fertilizer or soil lab that uses dedicated

elemental combustion-technology analy-

zers can also maintain an advanced ICP-

OES analyzer as a reliable backup. If the

elemental analyzer should fail, the ICP-OES

instrument can produce good results on

critical elements such as nitrogen (N), sulfur

(S), and phosphorus (P).


Calibration and standardization

Fertilizer lab analyzers should also supply

superior ease of use. For instance, during

calibration, SPECTRO ARCOS provides

simple tools to enhance accuracy. When

an aqueous calibration doesn’t match the

control sample from a manufacturer’s

certified reference material, the software

offers an additional correction model. The

user can just employ this sample as a type

standard, thus easily correcting the bias —

and enhancing measurement accuracy.

The fertilizer industry as a whole is in the

process of creating accepted standards

and methods for ICP-OES analyzers. The

Association of American Plant Food Control

Officials (AAPFCO) is developing a method

for analyzing trace impurities, using an

advanced SPECTRO analyzer. And the

Association of Analytical Communities

(AOAC) committee has an experienced

SPECTRO specialist on its Methods Forum.

Conclusion

Agronomic analyses for materials such

as soils, plant tissues, and fertilizers can

present considerable challenges, even for

today’s spectrometers. Many analyzers

currently on the market may fall short of

satisfactory performance.

Fortunately, advanced ICP-OES analyzers

such as SPECTRO ARCOS provide features

including superior ORCA optics, no-

compromise plasma viewing, and full

forensic software. So all types of labs can

find required levels of performance for even

the most challenging agronomy analyses.

GERMANYSPECTRO Analytical Instruments GmbHBoschstrasse 10D-47533 KleveTel: +49.2821.892.0Fax: [email protected]

U.S.A.SPECTRO Analytical Instruments Inc.91 McKee DriveMahwah, NJ 07430Tel: +1.800.548.5809 +1.201.642.3000Fax: [email protected]

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High-Performance Agronomy Analysis Using ICP-OES · 2020. 7. 8. · physiology, soil fertilization, and soil science — improves the use of soil and increases the production of food