high speed mass spectrometer as a gas chromatography detector

8/12/2019 High Speed Mass Spectrometer as a Gas Chromatography Detector

1/16

Section

V

Paper 16

u. c

A

HIGH SPEED M SS SPECTROMETER

S G S

CHROM TOGR PHY DETECTOR

IN THE DETERMIN TION

O

CRUDE OIL COMPOSITION

V. F Gaylor

-

C.

N.

Jones

**

-

J.H.Landerl

***

-

E.

C. Hughes

****

Abstract,

A

rapid crude oi l analysis procedure providing

boiling range data to 725 F and naphtha composition

in less than two hours has been devised. Whole crude is

charged to a gas chromatography unit comprising a strip -

per column and an analytical column.

A

t ime-of-f l ight mass spectrometer is used as an auxi l iary

detector for a conventional gas chromatography appa-

ratus. Continuous spectral monitoring of the column ef-

f luent permits identi f ication of single chromatographic

peaks. The spectrometers

1

microsecond scan rate is

particu larly valuable for studying distribu tion of indiv idual

compounds in non-homogeneous multicom pone nt chrom-

atographic peaks.

The mass spectrometer detector is used to characterize

chromatographic separations of virgin naphthas boi l ing

to 400 . Detailed spectra l analyses pro ve that the poor-

ly resolved peaks and shoulders obtained fro m a packed

chromatographic column could be related to hydrocarbon

type composition.

A

simple analyt ical procedure for estimating cycloparaf-

f in content from chromatograms of crude oi l and reform-

ing feed results in a method that is suitable for routine

refine ry use.

Rsum.

Larticle dcrit une nouvelle mthode danalyse

rapide du ptrole brut, indiquant les points dbullition

jusqu 725

F

et la composit ion des naphtas en moins

de deux heures. On introduit le brut entier dans une

unit de chromatographie gazeuse comprenant une co-

lonne de sparation et une colonne danalyse.

On monte un spectromtre de masse

temps de rponse

ultra rapide uti l is comme dtecteur auxi l iaire, sur un

appareil classique de chromatographie gazeuse, Le con-

trle spectral continu de l eff luent de la colonne permet

diden tifier chaque pic chroma tographique simple et, grce

l

vitesse de balayage du spectromtre

100

micro-

secondes),

l

est possible de dist inguer la rp art i t ion des

diffrents individus dans les pics complexes non homo-

gnes.

On uti l ise le dtecteur de spectromtrie de masse pour

caractriser les fractions de naphtas vierges ayant jus

qu

400

o F 205 C) de po int dbu llition initialemen t

spares par chromatographie. Des analyses spectrales

dtai l les montrent qu i l est possible de rel ier les pics

mdiocrement

rsolus

et les sai l l ies peu diffrencies

fournis par une colonne chromatographique garnissage

une composition type dhydrocarbure.

Une mthode danalyse sim ple perm ettant dvaluer la

teneur en cycloparaffines des bruts et des charges de

reforming, partir de leurs chromatogrammes, offre un

outi l appropri aux besoins usuels des raff ineries.

Introduction

The numerous applications of gas chromatography

to analysis of petroleum streams are well known.

Widespread, routine us s have, however, been

gener ally limited to the rel atively simple Ci through

C hydrocarbons. Outstanding successes in analyz-

ing more complex mixtures have been achieved,

using specialized techniques and equipment, but

often involve increased costs and analysis time.

* Authors Biographies v ide las t page

Methods which retain the speed and simplicity

features of the original chromatographic technique

may require a new approach to interpretation of

complex chromatograms. This paper describes such

an approach t o crude oil analysis, based

on

research

with a mass spectrometer used as a chromatographic

detector.

Previously reported applications of gas chromato-

graphy to quantitative crude oil analyses were

limited in scope. WebbI3 injected whole cr ude oil

directly into an analytical column and measured

Ci

through S hydr,ocarbons. Martin and WintersQ

201


2/16

and Rysse1berget2) used a short precolumn to retain

heavy ends and quantitatively determined Cz

through C7 and C2 through C5 hydrocarbons, respec-

tively. A temperature-programmed gas chromato-

graphy column was used by Barras and Boyle') for

predicting jet fuel freezing point from crude oil

analysis.

The temperature programming technique vastly

extends the useful molecular weight range of a single

chromatographic analysis. Calculation of crude oil

boiling range distribution from a temperature-pro-

grammed chromatogram seemed feasible. Eggertsen,

Groennings and Holst5) conver ted programmed

chromatographic analyses of petrole um fractions to

distillation-type curves which closely approximated

a 60-plate distillation; Barras and Boyle1) used

analysis on an uncoated Chromosorb column to

predict yield points on a l-plate distillation column.

While distillation-type curves we re of obvious

value, determination of chemical composition from

crude oil chromatograms was equally desirable.

Chromatographic procedures were designed to

optimize both molecular weight range and s epara tion

efficiency in a single analy sis. Normal tricosane was

eluted in abou t 35 minutes while cons iderab le resolu-

tion of low molecular weight hydrocarbons was

retained.

As

many as 50 distinct peaks and shoulders

were obtained in th e C3 through Cii range, constit ut-

ing a fingerprint profile qualitatively useful for

characterizing crude oil. Quantitative interpretation

in terms of single compounds or over-all hydrocar-

bon type distribution require d some knowleld,ge of

composition of each single peak.

Identificationof the C3 through C7 peaks presen ted

no particular problems. Identification and determina-

tion of single compounds in the Cs-Cii range is more

difficult because of increasing complexity of composi-

tion. Desty, Goldup and Swanton3) identified more

than half of 122 peaks resolved from the C3-Cn

portion of the American Petroleum Institute 's Ponca

City crude in a 20-hour run on a coated capillary

column. Polgar, Holst and Groennings ) accom-

plished complete analys is Of C7 and CS alkanes, cyclo-

pentanes and cyclohexanes with two successive an-

alyses on a 300-feet capillary column totalling about

3 hours;

80

O o

of the possible compounds were quanti-

tative ly accounted for a s individuals. Lindeman and

Anni@) use'd a magne tic-type mass spe ctrometer

for complete qualitati ve and quanti tative analys is of

chromatographic pea ks of a

440

F

end-point naphtha

on a packed column; over 60 single compounds were

identified and general structure assignments wer

report ed for many Ci0 and Cii compounds.

It was unrealistic to expec t that single compoun

ana lyses could be achieved chromatographically from

the

30

poorly separated peaks representing th

hundreds of

c

through Cii compounds likely presen

in crude oil. Consistency of the peak patt ern, how

ever, and large variations in pea k size distribution

from sample to sample suggested that gross hydro

carbon -type distribution a nalyses might be compute

direc tly from the chromatogram. Peak height measu

rements were preferred for this purpose since appa

rent separation even between discrete peaks wa

usually poor. Qualitative composition analysis a

each peak maximum was thus desired. Time-O

Flight mass spectromete r, which produces 10,00

spectra per second, had the required speed an

sensitivity for monitoring chromatographic colum

effluent )

4

6 .

Photographic recording of spe ctr

permits qualita tive composition determination at an

single instant during elution of the chromatographi

peak. Mass spectra obtained in this way we re use

to estimate rela tive hydrocarbon-type distribution a

each peak maximum.

quipment and Procedures

A commercial gas chromatography unit (Mod

K-2 Kromo-Tog) employing a filament-type therma

conductivity detector,and equipped with temperatur

programmer, flow controller and precolumn assembly

was used (Burrell Corporation, Pittsburgh, Pennsyl

vania) . Primary elements of t he chromatographi

equipment ar e adequately described in the manufac

turer's literature. Details of the conventional flow

system are depicted in Figure 1 and are largely self

explanatory. The hairpin-shaped analytical colum

was 250 cm. long and 5 cm. I.D. The V4-inch I.D. pre

column was one foot long and contained packin

identical

to

the analytical column. Primary helium

carrier was directed through the precolumn durin

sample injection and maintained until desired por

tions of crude oil had been flushed into the analytic a

column. Carr ier flow was th en switched to precolum

by-pass position. Crude oil heav y ends trapped in th

precolumn were backflushed and vented to atmos

phere at the flash vaporizer inlet port.

202 VI1


3/16

FIGUR

FLOW

S Y S T M

FIGUR

FLOW

S Y S T M

Two di fferent column packings an'd corresponding

experim ental parameters wer e used, Table I. Highest

molecular weight range w as achieved with Procedure

II while Procedure I was p refe rred for resolu tion of

low molecular weight hydrocarbons. Resolution of

Ci-Cs hydrocarbons was optimized in either proce-

dure by holding column temperature a t 25' C until

n-C5 was eluted.

Column packings w ere tempera ture stabilized be-

fore use b y heating for 24 hours under helium flow.

Columns I and II were programmed to 250'C and

300' C, respec tively, without detectable liquid phase

bleeding. Maximum molecular weight determined

was Cis for Procedure I and Ce3 for Procedure II.

Chromatograms were recorde d on a 1.0 millivolt

Potentiometer recoilder (Minneapolis-Honeywell Reg.

Co., Philadelphia, Pennsylvania), equipped with Disc

integrator (Disc Instrument Company, Santa Ana,

California) for peak area dete rmination .

The Time-Of-Flight mass ~pectrometer'~)

5)

Model 12-100 (The Bendix Corporation, Cincinnati,

Ohio), was attached to the chromatographic equip-

TABLE I

CHROMATOGRAPHIC OPERATING PAR-TERS

Procedure I Procedure

II

Column Packing

Analytical Column Temperature:

Initial Temperature.

.........................

Initial Temperature Hold.

Heating Ra te .. .............................

Final Temperature ..........................

Precolumn Temperature.

Precolumn

Flush

Time

.........................

Flow

Rate.. .................................

Sample Size

5.0 wt. yoApiezon L

on

30-60

mesh ChromosorbP

25

C

3.0

minutes

7 C p r

min

250

C

200 c

3 minutes

5 OB per

min

.o12 ml.

1.0 wt. yo D.C. 710 Fluid

and 3.0 wt. Apiezon

L

on 40 50 mesh

Chromosorb

P

25

C

2.0 minutes

11

C

per min.

300 c

300

c

1

minute

230

cc

per min.

.O24 ml.

ment at the detector vent (Figure

1).

The variable

leak (Model 9101-M, Granville-Phillips Company,

Pullman, Washington) was adjus ted to raise internal

spectrometer pressure to 2X 10 mm. mercury with

pure helium effluent. Temperature of the

2 5 VE

I.D.

line connecting the leak to the spectrometer inlet

port was maintained at 170' C and the spectrom eter

source chamber was hea ted to about 175OC. The

ionizing source was ope rated at

2.5

amperes filament

current, .125 microamperes trap current and O e.v.

energy level.

Spectra were viewed on th e screen of a Type 541

A

oscilloscope, equipped with Type CA Preamplifier

(Tektronix, Incorporated, Cleveland, Ohio) and pho-

tographe8dwith a Polaroid camera. Relative mass line

intensities were

estimated by use of a Microcard

Reader (Microcard Reader Corporation, Wes t Salem,

Wisconsin).

V/16 2 3


4/16

Discussion

boilin,g range yield point calibration. The columns

described separated paraffins essentially by boiling

point and exhibited only slight selectivity for naph-

thenes and aromatics. Naphthenes were retarded,

Normal paraffin peaks in the crude oil chromato- relative to paraffins, by about one peak width and

grams were readily identified by either relative size aromatic selectivity was only slightly greater.

or elution time (Figure 2) and were employed for

Chromatographic analyses of carefully fractionated

Boiling Range Distribution Analysis

FIGURE

CR UDE

O I L

C H R O M A T O G R A M P R O C E D U R E

distillates of both highly aromatic and naphthenic

crude oils showe d that error due to column selectivi ty

was negligible. Consequently, norm al paraffin boiling

point-elution time plots were used for yield point

calibration.

Liquid volume per cent distillation yields were

oalculated .directly from summed peak areas. Mess-

nerlO)showed that thermal conductivity molar area

response for hydrocarbons is a function of both

molecular weight effects. Normal paraffin area

for single compounds thus requires accurate know-

ledge of re lat ive response of each compound. Aver-

aging structure effects seemed permissible, however,

when summing >areas of peaks composed of many

hydrocarbons likely including most possible struc-

tures.

Quantitative oalibration with normal paraffin

standa rds was a rea dy means of compensating for

molecular weight and structure. Quantitative analysis

response per un it liquid volume sampled also tended

to average out structure effects at each carbon

number level (Figure 3 . Liquid volume per cent

yields were therefore calculated from arealvolume

constants experime ntally de termined for each C5-

C 3 normal paraffin. Cons tants for C4 and lighter

NORMAL PARAFFINS

OIS

FWUFFINS

xNAPHTHENES

AROMATICS

FIGURE

EFFECT OF H Y D R O C A R B O N T Y P E O N

LIQUID

V O L U M E R ES P O N S E

paraffins were obtained by extrapolating linear

molar response curves.

2 4

Vi

6


5/16

Composition o f

L i gh t N a p h t h a a nd O f f - G a s

Rresolution of low boiling hydrocarbons was suffi-

cient to obtain considerable composition information

from the crude oil chromatogram (Figure

4 .

Complete

separa tion of propane, isobutane and normal butane

was not achieved; resolution was, howeve r, adequate

for estimating concentration s with reasonable

degree of accuracy.

Octane number of light naphtha boiling 55-220

F

was estimated from relative concentratio ns of normal

paraffins and summed isoparaffins, naphthenes and

benzene. Weighted averag e blending octane numbers

of 45.0 for normal paraffins and 79.5 for isoparaffins,

naphthenes and benzene, calculated from pure

compound octane numbers determined by the Ameri-

can Petroleum Institute s Projec t 45, were used for

octane numbers prediction. Values estimated in this

way were at least

as

accurate a s values calculated

similarly from single compound analyse s.

FIGURE

P A R T I A L C R UD E O I L C H R O M A T O G R A M

P R O C ED U R

E

LE G E N D : 1 M E T H A N E N O N - C O N D . G A S E S

2.

E T H A N E

3

P R O P A N E

4

I S O - B U T A N E

5 N - B U T A N E

6 . I S O - P E N T A N E

7. N - P E N T A N E

Peak Label l ing

As many as 30 major peak s and shoulders were

obtained i n the n-CI to n-Ci1 portion of the chromato-

gram. Peak pattern could be considerably altered by

relatively small changes in column packing, as, for

example, Column I

vs

Column II (Figures 2 and 4 .

However, with identical ana lytical procedures, the

over-all peak patte rn was consisten t for many differ-

ent crude oil types. Most of the composition analyses

wer e obtained from Column though the same pro-

cedure s and principles w ere also effective for Column

II.

Chromatographic peaks eluted between n-Cs and

n-Ci1 w ere labelled for da ta hand lingpurposes. Major

group numbers were assigned to each series of pe aks

eluted between successive normal paraffins. Group

8

peaks included all peaks eluted between n-C7 and

n-CS, Group 9 peaks were eluted between n-Cs and

n-Cg, etc. (Figure

4 .

ingle peaks in each group were

VA

205


6/16

further identified by elution order with in the group.

A partia l relative retention time PRRT wa s calculat-

ed for each peak:

I

where Peak X is eluted between n-Pi and n-Pn.

The peak number identification included both group

number and PRRT anmdhus .defined exact location on

the chromatogram. For example Peak 8-50 was

eluted exact ly halfway between n-C7 and n-Ca.

Group number does not necessarily define carbon

number. The peaks immediately preceding a normal

paraffin of ca rbon number N usually contained both

and N 1 naphthenes and aromatics and isoparaf-

fins largely of N carbon number.

i l l

a s s

Spectrometer

nalyses

1

Useful mass spectr a

of

single peaks were achieved

only by minimizing lag time between chromatogra-

phic and spectrometer units. Careful attention to

length and temperature of

low

pressure inlet lines

produced virtually simultaneous response from both

ddtection systems. Over-all spectrometer speed and

sensitivity were sufficient to permit qualitative

characterization of small shoulders immediately fol-

lowing or preceding major peaks. For example

spectra of Peak 9-.69 show ed the peak like ly

contained a large amount of 3-methyl octane while

the 9-.75 shou lder wa s largely composed

of

ethyl

benzene and one or more Cg naphthenes Figure 5.

m

98

m e

n

I

t

z

39

4

43 55 57 67 69 7 81

8

9 97 1 6 126

mie

m

e

FIGURE

M A S S S P EC T RA L C H A R A C T E R I Z A T I O N

OF

A D J A C E N T

P E A K S

Contribution of 3-methyl octane to spectra of t he

9-.75 shou lder was small. Similarly the naphthe nic

character of the 10 .45 shoulder was not obscured

by paraffincontribution from the larger

10 .61

peak.

Spectral comparisons of most poorly separated

peaks showed chromatographic column efficiency to

be a good deal better than was superficially apparent.

Expected non-homogenity was detected and often

explained occasional anomalous peaks resulting from

large variations in single compound concentration.

Xylene and Cg naphthe ne for example wer e often

detec ted spectr ally in the leading edge of n-Cg peaks

Figure 6 through not evident chromatographically

A

shoulder on the n-Cg peak sometimes observed in

206 V 16


7/16

consistent with boiling point relations between

branched and normal paraffins of the sam e carbon

Identification of single compounds, though possible

in many instances, was not a primary aim. Mass

spectra were used semi-quantitatively to estimate

hydrocarbon-typedistribution-ateach pea k and shoul-

number.

h

SAMPLE A n Cg

PEAK

FIGURE

der maximum. Repeatability was about rf

3

O o and

accuracy was largely unknown. Spectral analysis

could not necessarily define absence

of either naph-

thenic or paraffinic hydrocarbon, as was the cace for

aromatics. Apparent detection of small amounts of

paraffin in a predominantly naphthenic peak, or con-

versely, could have resulted from spectral inaccura-

cy, contamination from an adjacent poorly separa ted

peak, or could have been real. Hydrocarbon-type

distribution est imates were used only for qualita-

\

J

SAMPLE

B

n C9

tive guidance an,d inaccu racy was of no particu lar

concern.

PEAK

X Y L E N E D E T E C T I O N I N

TWO

CRUDE OI LS

M J O R X Y L E N E ml LI NES =

91

92 105

106 107

chromatograms of more aromatic crude oils, was

largely composed of xy lene wi th smaller amounts

o normal nonane and g naphthene.

Mass spect ra of compara ble chromatographic

peaks were qualitatively identical from sample to

sample. Comparative spectra contained the same

mass lines though relative intensities varied some-

what with differences in single compound or com-

pound-type distribution within a peak. Spectra of

the 9-.12 peak in paraffinic crude oils, for example,

indicated approximately equal concentrations

of

paraffinic (isoalkanes) and naphthenic (cycloalkanes)

compounds while naphthenes predominated in the

same peak in naphthen ic crudes, Figure 7.

Many

of

the chromatographic peaks were, how-

eve r, consist ently pure in hydrocarbon-type com-

position. Mass spectra of 9-.62 peaks were predo-

minantly paraffinic and 10-.10 shoulder spectra

were highly naphthenic in both paraffinic and na ph-

thenic-type crude oils, Figure

7.

In ge neral, isoparaf-

fin s were concentrated in the chromatographic PRRT

ranges of

.6

to

7

and

.2

to

3

in each peak group.

Conversely, in the PRRT ranges 1 to

.2 .4

to 5 and

.8

to

.9,

isoparaff in levels w ere minimum and naph-

thenic spectra were usually observed. This PRRT

distribution of isoparaffins in each peak group is

Quantit ative Hydroc arbon-Type Distribution

Accurate naphthene contents of peak group frac-

tions, collected from a p repa rative scale ga s chroma-

tography unit, were determined by conventional

mass spect rometry. Direct comparisons of composit-

ed a nalytica l ,data showed tha t th e chromatographic

peak profile could be related to gross hydrocarbon-

type distribution. Relative heights of predominantly

naphthenic peaks were directly proportional to total

naphthene content in each group, Figure 8. The major

naphthenic peak was larger tha n the major isoparaf-

fin peak in each group fraction of the Illinois crude

oil; specifically, 7-.27

>

7-.74

8 .30

>

6 . 6 4

9-.47

>

9-42 10-.45 > 10-.61 and 11--48

>

11-62. The reverse was true for the more paraffinic

East Texas crude;

7-.27

7-.74 8-.30

high speed mass spectrometer as a gas chromatography detector

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