Adlard E.R. (ed.) Chromatography in the Petroleum Industry

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Chapter

lo000

lo00

100

10'

F=1/31

F=l/lI

1/46

112.7

-__-

---

....

.....

Slit

width

JJITI

Fig.

7.1

Selectivity

for

chlorine

function

slit

width.

The selectivity improves with decreasing slit widths, as shown in Fig.

7.1

for

the C1479.4 nm emission line. There is not a perfect linear relationship between

slit width and selectivity because of the limited resolution

the monochroma-

tor.

the slit width

also

has consequences for the

MDL,

a compromise must be

made between MDL and selectivity. Without means to improve the selectivity,

our

choice was generally a 50pm slit, because there

only a small loss in LDL

and twice the selectivity compared to a l00pm slit. In general, the selectivity

ratios for the

LPP

are somewhat better than for the

APP.

With the triple-slit exit system, the selectivity ratios are increased by a factor

30-200.

The precise increase in selectivity depends upon the accuracy with

which the compensation

for

the background variation is adjusted. This adjust-

ment can be made by injecting a pure hydrocarbon, while monitoring both the

element line emission and the background. Within the linear range

the detec-

tor, there is a constant ratio between the

signal of the background and the

signal in the element line. In a

analysis, both line emissions are monitored

and after the analysis, the complete chromatogram

the element line is cor-

rected for the background. When the correction factor

known, it is also possi-

ble to correct the element line for the background continuously.

When the plasma is overloaded, positive and negative peaks can occur after

correction; moreover the element signal may no longer be proportional to the

element concentration.

TABLE

7.6

PERFORMANCE

OF PLASMA

DETECTORS

Element Line Low-pressure plasma Atmospheric-pressure plasma

(nm)

MDL MDL Sel. Sel. LDR

MDL

h4DL

Sel.

LDR

(ppm)

(PP/s)

uric-

xl000 xl000

(ppm)

(pgk)

un-

1000

con.

corr.

con.

241.8

0.006

300

0.003 0.5

193.1

0.001

0.2

1000 0.001 0.2

200

656.2

0.008

0.1

800

(200) 0.03

0.4

270

(20)

486.1

0.04

0.6 250 20 (40) 0.17

2.4 70 20 (4)

656.4

0.008

0.2 600

(200) 0.04 1.1 100 10

771.2 0.13 29 40

0.038

8.4 10

146.9 0.13 25 1300 10 40 0.3

750 10 2

479.5

0.009

4.4 940 100 100 0.012 5.9 950 100

470.5 0.006 6.7 1600 40 200 0.005 5.6 620 20 20

516.1

0.008

14 1040 40

200

0.004 7.1 210 20 20

685.6 0.07 19 740

0.08

22 400

545.4

0.05

22 180 20 20 0.06 21 120

180.7 0.01 4.5 850

100 0.01 4.5

500

365.0 0.0002 0.6 105 >lo00 1000 0.0001 0.2 105 >loo0 1000

750.3

0.03

17 2900 100 0.2 100 100 7

473.8 0.03 40 460 20 50 0.15 160 140 10 4

529.2

0.008

13 4200 100

0.025 29 1200 100 3

640.2 0.05 14 540 50 100 0.06 17 460

184

Chapter

7.5

CONCLUSIONS

Table

7.1

presents the conditions used for the evaluation of both types of

plasma. A summary

the results

given in Table

7.6.

For

comparison, for the

MDL, the results are also given in pg/s. From these results, we may draw the

following conclusions:

The

MDLs

for both types of plasma are the same. The

MDLs

are between

0.1

and 100 pgs, depending on the element to be measured and on the in-

tensity of the emission line used. The MDLs of the atmospheric plasma are

almost the same as the data presented by Quimby

al.

[23].

order to obtain the same

MDLs,

the light-collecting system of the low-

pressure plasma must be better and

more expensive.

The maximum concentration that can be introduced into the low-pressure

plasma is about ten times that

the atmospheric plasma.

The linear dynamic range is between

lo3

and

lo5

for the atmospheric- plasma

and between

lo4

and

lo5 for the low-pressure plasma.

The selectivity ratio with respect to carbon is about the same for both plas-

mas and ranges from

100

2000

without a correction system in use. With

corrections (triple-slit system) the selectivity ratios relative to carbon are

between

lo4

and

lo6.

Because of the good sensitivity and very short. response times, both types of

plasma detectors are well suited for use with capillary gas chromatographic

columns.

7.6

DESCRIPTION OF THE HEWLETT PACKARD

5921A

AED

Apart from the plasma emission detectors described in the previous

paragraphs, supplied by Applied Research Laboratories, and then by Applied

Chromatographic Systems, a number of other plasma emission detectors

have been announced by other instrument manufacturers. At the 1989 capil-

lary chromatography symposium in Riva del Garda, Italy, the Varian Instru-

ment Group (Walnut Creek, CA,

USA)

announced an atomic emission detector

[27]

and in May 1990 Carlo Erba Instruments (Rodano, Italy) announced their

plasma emission detector ESD-4

[28]

but to the time of writing, these detectors

have

not

been released. The only plasma detector that is commercially available

at present is the Hewlett Packard (Avondale, PA,

USA)

5921 atomic emis-

sion detector (AED) and this instrument is described

more detail in the next

sections.

Microwave plasma detectors

185

7.6.1

Gas chromatograph and transfer line

The gas chromatograph most convenient for coupling to the AED is the

5890A series

adapted with a transfer line between

and the AED cavity. It

has a mode of monitoring and controlling the flow through the column by pres-

sure programming. The complete set-up

this

and

AED

depicted in Fig.

7.12.

Since the AED is a concentration-type detector, this constant flow mode of

operation is mandatory for quantitative work. The transfer line consists

1.5 m

of heated and insulated outer aluminium tube, with an inner tube of stainless

steel with an i.d. just over 1 mm,

that it heats the analytical column inside

properly. Since heating up this transfer line is a rather slow process, it should be

kept continuously at maximum column temperature. Up to the normal operating

temperature of

300"C,

no specific measures have to be taken. In high-

temperature

GC,

however, where temperatures far over

300°C

are quite com-

mon, the limit of the temperature specification (which is

400°C)

is reached for

the transfer line. Neither polyimide-coated fused silica columns, nor aluminium-

clad columns can withstand these temperatures for more than

days (which is

50 h continuously). The only columns capable of being bent (to be pushed

I-

I----

__-

--A

!TI;

Fig.

7.12.

Diagram

the Hewlett Packard GC-AED:

(1)

autosampler;

(2) GC;

(3)

capillary col-

umn;

(4)

transfer line;

(5)

cavity;

(6)

magnetron; (7) spectrometer;

(8)

water cooling pump;

(9)

cavity

gas

control;

(10)

computer.

References

200

186

Chapter

through the transfer line) which can withstand these temperatures for a long pe-

riod of time are stainless-steel high-temperature columns. Experiments have

shown that these columns only have to be replaced after

month of operation.

This means that the last

1.5

the column has been at

400°C

for

700

con-

tinuously! These columns are available from Chrompack, Middelburg, The

Netherlands. The gas union

the cavity, which

the exit of the column to the

detector, as is shown in Fig.

7.13,

has an i.d.

0.75 mm, since it was designed

for acceptance

of polyimide capillary columns. The steel capillary columns,

however, have an 0.d. of

0.8

mm,

the gas union should be drilled out to about

0.9

mm to accept these columns.

Fig.

7.13.

The

atomic emission detector:

(1)

plasma;

(2)

capillary column;

(3)

make-up and

reagent gasses in-out, solvent vent;

(4)

discharge

tube;

(5)

cooling water;

(6)

microwave energy in;

(7)

optical window;

(8)

window purge;

(9)

O-ring;

(10)

heated zone;

spectrometer.

Microwave plasma detectors

187

7.6.2

Microwave cavity, discharge tube and gas flow system

The cavity resembles the Beenakker type, except that it has a pedestal in the

centre and a smaller diameter

[23].

Microwave power is supplied to the cavity

through a waveguide of

mm by

mm,

on which a microwave oven magne-

tron tube is mounted. The

cavity/waveguide/magnetron

assembly is attached to

the spectrometer in such a way that it allows the cavity to be swung away for

discharge tube replacement and repositioning. The cavity contains a quartz dis-

charge tube of

1.0

i.d.

1.25

mm 0.d.

long, which is water-

cooled to eliminate erosion. The lifetime of the discharge tube as measured, is at

least

month. Since columns were replaced every month, and replacement of the

column can only be performed by disassembling the heated part of the cavity, the

discharge tubes were replaced at the same time. The exit of the cavity is closed

with a UV-grade fused silica window and purged with helium to prevent back

diffusion of ambient air. It also enables reversion of gas flows inside the dis-

charge tube to allow solvent venting.

The cavity contains a

cartridge heater, enabling heating of the cavity

block up to 400°C. Coupling the analysis column to the cavity block is per-

formed through the gas flow system. This system allows the entrance of one or

more auxiliary gases (He,

H2,

02,

N2,

CH,),

or venting of sample solvent, as

controlled by the computer. The use of these gases prevents peak tailing greatly,

depending on the nature

the peak.

recipe for the proper use of these gases is

incorporated in the computer software.

When a sample is to be injected by the on-column injection technique, it gen-

erally has to be diluted before injection, The amount of diluted sample injected

into the column will then be in the order of a few microliters, and this large

amount of solvent will extinguish the plasma, or even worse, cover the inner sur-

face of the discharge tube with a layer of carbon deposit. This layer of soot can

only be removed by the plasma itself, which takes many hours. For these analy-

ses, a solvent vent is a very convenient part of the design. The choice made in

the design of this detector to cool the discharge tube with water, instead of a gas-

cooling system, has some important practical consequences (see Fig.

7.14

for the

detailed construction):

prevent a significant consumption (and thus loss)

microwave power

by the cooling water, the water film is only 100,um thick. This calls

for

filtering of any particulates out of the water and prevention of the forma-

tion of algae,

a water-filtering system is essential.

cool the discharge tube effectively, the linear velocity of the very thin

water film must be rather high. Together with the filtering system, it de-

mands a powerful pump and a flow sensor to stop the microwave power if

the flow drops below a predetermined value.

References

200

I88

Chapter

Fig.

7.14.

Construction of the

AED

cavity with water cooling:

(1)

pedestal;

(2)

discharge tube;

(3)

quartz

jacket;

(4)

water in;

(5)

out;

(6)

polyimide ferrule;

(7)

cover plate;

(8)

makeup and rea-

gent

gas

inlet and vent outlet;

(9)

heat

stand

(10)

heated zone;

column;

(12)

column

fer-

rule;

gas union;

collar;

water plate;

(16)

coupling loop.

those cases where high column temperatures are necessary, there is a

rather large temperature difference between the gas union and the water

plate. This temperature drop is effected over the polyimide ferrule, which

has a triple function: (a) it isolates the water system from the gas system;

(b) it isolates the cavity overpressure from the outside; and (c) it fixes the

discharge tube to be kept in place. The demands put on the ferrule are ex-

cessive when temperatures over 300°C are needed.

The coefficients of expansion of the stainless steel of the gas union and the

polyimide

the ferrule are quite different. When using temperatures over

300°C

and switching off the block heater, which is e.g. performed by the flow sensor in

the case of too high a restriction

the water lines, the ferrule will crimp more

than the gas union. Cooling water will commence to leak along the ferrule and

sometimes into the electronics compartment. Complete disassembly of the cav-

ity,

replacement of the ferrule and repositioning of the discharge tube are then

necessary. The polyimide of the ferrules in use for more than one week at

400°C

appear very much deteriorated at inspection. After installation of a fresh ferrule,

the cavity needs partial re-assembly after a day, to retighten the ferrule. A redes-

Microwave

plasma

detectors

189

ign of this part of the detector for high-temperature work should therefore be

considered.

7.6.3

The photodiode array spectrometer

The light emitted by (the elements in) the plasma passes a helium-purged

quartz window. It passes the

0.05

mm entrance slit of the spectrometer and is

diffracted and focussed by a concave holographic grating with a flat focal plane

0.35

m and sent to a

pixel photodiode array (PDA)

[24].

This PDA moves

along the focal plane, which is nearly linear in shape from

160

800

nm. The

optic arrangement is depicted in Fig.

7.15.

Simultaneous measurements

lines

more than

10-30

nm apart (depending upon their position in the focal plane) is

impossible. Multiple chromatographic runs are necessary in those cases.

The spectrometer is capable

measuring clustered lines within the width of

the PDA simultaneously. But it can also be set at any wavelength and

can scan

a continuous spectrum. It includes both polychromator and monochromator fea-

Fig.

7.15.

Optics schematic diagram

the

AED:

(1)

helium plasma;

(2)

90'

elliptical mirror;

(3)

entrance slit;

(4)

concave diffraction grating;

(5)

PDA,

(6)

focal plane.

References

200

190

Chapter

tures. The spectrometer itself is heavily thermally insulated and since wave-

length correction is performed at the start of each chromatographic run, it means

that the thermal time constant of wavelength precision

in excess of

h. The

cavity

attached to the outside wall of the spectrometer,

that the focus of the

elliptical mirror

into the end

the discharge tube. The 21

detecting

elements of the PDA are all active

100% of the time,

that signal and back-

ground portions next to a spectral line are continuously measured and corrected

for.

7.6.4

Computerized control and data treatment

The gas chromatograph as well as the AED is computer controlled with a HP

35920A

GC-AED Chemstation. All functions of the system can be performed by

softkey control. Correct scavenger gases are automatically turned on, and the

required order sorter (optical filter) is chosen. It automatically calibrates the

AED

for

wavelength determinations before each run.

To locate the atomic line centres with the PDA precisely, the centre of gravity

calculation algorithm

[29] is used.

For

most atomic lines, it results in a standard

deviation of the estimation of the line centre better than

0.01

pixel. The helium

spectrum provides several narrow, intense lines most suited as wavelength stan-

dards. The auxiliary gases provide small amounts of carbon, nitrogen, hydrogen

or oxygen, which also can be used as calibration lines. The mechanical position

the PDA on the focal plane is converted to nominal wavelength (which is very

linear), using a numerical model of the spectrometer. Since selectivity

non-metallic elements

atomic emission spectroscopy relative to carbon rarely

exceeds

few hundred, background correction is necessary to improve this. An

improvement by several orders of magnitude is obtained by means of real-time

multipoint background correction. See also Table

7.7.

The PDA read-out time is usually 10

Hz,

but sometimes

100

is used during

calibration routines. Signals specific for

element are produced by a computer-

induced “recipe”, composed of a pair of linear functions of the various pixel sig-

nals.

this recipe, pixel responses, element filter and background signal are re-

corded,

that the calculated background amount to give optimal selectivity can

ad-justed post-run.

7.6.5

Characteristics

The sensitivities and the selectivities of the various elements depend on the

element line used for measurement.

number of common elements on which we

Microwave

pfasma

detectors

191

TABLE 7.7

SENSITIVITIES

AND

SELECTIVITIES

SOME

COMMON ELEMENTS WITH

THE

HEWLE'IT PACKARD AED

~ ~~

Element Wavelength DL (Pma Selectivity (relative to carbon)

Nitrogen 174 10

Sulphur 181 <2

Carbon 193 <1

Carbon 248 2

Hydrogen 486 2

Carbon 496b 10

6000

150000

aTo enable comparison

the

AED

with e.g. the FID, the MDL is expressed in pg/s, where the

AED

is a concentration-type detector and the MDL should basically be expressed in a concentra-

tion unit.

bAlthough carbon 496 is a 2nd order line

carbon 248, it is generally used when both hydrogen

and

carbon have to be determined.

determined these characteristics, all exhibited better selectivities and sensitivities

than the specifications, as supplied with the instrument, suggested. Some typical

examples are given in Table

7.7.

7.7

TYPICAL APPLICATIONS

7.7.1

Multi-element

SimDist

analyses

The use of gas chromatography for simulated distillation (SimDist) analysis

and its implications are described elsewhere in this volume. For the specific

analysis

multi-element SimDist, especially on residue-containing products,

some practical considerations are discussed.

To enable the elution

very high-boiling material from the column, short

(typically 5-10 m) wide-bore capillary columns are used. These columns contain

a very thin film (typical 1100 nm)

a chemically bonded, non polar stationary

phase, stable at high temperatures (typical

44OOC).

mentioned in the previous chapter, the AED is a concentration-type detec-

tor and quantitative results can only be obtained when the flow is strictly con-

stant. The

flow

mobile phase (helium) through the column

generally about

ml/min, which means that the column pressure drop is around 15 Pa. Since

the cavity pressure, caused by auxiliary gases and window purge gas, is about

Ha, the column head pressure is about twice the pressure drop of the column.

The

5890A

series

gas chromatograph is equipped with a so-called con-

stant-flow module. However, this constant flow

obtained by pressure pro-

References

200