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Matrix Compounds for Fast Atom Bombardment
Mass Spectrometry
John Leveson Gower
Beecham Pharmaceuticals Research Division, Brockham Park, Betchworth, Surrey, RH3 7 k l , UK
The increasing number of applications for fast atom bombardment(FAB) mass spectrometry has seen a corresponding
increase in the use of different matrix compounds. This review discusses briefly the use of matrix compounds in
general and the use of cosolvents and/or additives with glycerol that assist in producing superior FAB mass spectra
than when glycerol is used on its own. The main part of the review deals with alternative matrix compounds to
glycerol including thioglycerol, polyethyleneglycols, triethanolamine, diamylphenol, crown ethers and various
miscellaneous matrix compounds.
The technique of fast atom bombardment (FAB) mass
spectrometry has become the most widely used of the
so-called ‘soft ionization’ mass spectrometric techniques
for the analysis of compounds not amenable to conventional electron impact or chemical ionization mass spectrometry. The technique of FAB mass spectrometry has
been discussed by Barber and coworkers.’,’ More specifically, Martin et aL3 have discussed the optimization of
experimental procedures and Lehmann et aL4 have
reported on studies concerned with some of the basic
aspects of FAB mass spectrometry, including matrix
effects, sample effects, sensitivity and quantification.
Przybylski5 has compared in detail the techniques of
FAB and field desorption (FD) mass spectrometry.
The exact mechanism of ionization occurring in the
FAB experiment remains uncertain. Whether it is wholly
a surface technique6 or whether there is a direct contribution of sputtered ions from the bulk remains ~nclear.’,~
The requirement for a matrix compound that enables
spectra to be generated over a relatively long timescale
is, however, well recognized as a fundamental aspect of
the technique.
Various different matrix compounds have been reported but glycerol remains by far the most widely used.
this review attempts to bring together information on
uses of alternative matrix compounds as well as discussing in general terms the purpose and requirements of
any matrix compound. A discussion on the use of
glycerol as a matrix compound is also presented. No
attempt to discuss in detail the mechanism of sputtering
is made, since although this is fundamental to the use
of liquid matrix compounds, it is outside the scope of
a review dealing primarily with applications of the use
of matrix compounds. It has also been decided to restrict
the review to practical examples of the use of various
matrix compounds and to leave a detailed comparison
of the physicochemical properties of these matrix compounds to a further publication.
The term fast atom bombardment is used here specifically to describe spectra obtained by sputtering from
liquid matrix compounds.
An extensive report on this work was presented at the
13th meeting of the British Mass Spectrometry Society
(Warwick, September 1983).
The extensive use of FAB mass spectrometry will no
doubt mean that many other matrix compounds will be
reported and it is hoped to publish further updated
reviews at regular intervals.
As already mentioned, the use of a matrix compound is
a fundamental aspect of FAB mass spectrometry and it
is well known that ion formation diminishes as the matrix
compound is depleted but that it can be increased again
by addition of further amounts of the matrix compound.
It seems from the experimental data that there are
three major requirements that a matrix compound
should fulfill viz:
It should dissolve the compound to be analysed
(with or without the aid of a cosolvent or additive),
thus allowing molecules of that compound to diffuse
to the surface layers replenishing the sample
molecules that have been ionized or destroyed by
interaction with the fast atom beam. An alternative
explanation for the mechanism of ion formation in
FAB mass spectrometry is that sputtering occurs
from the
rather than from the surface. If this
is so then solubility of the compound in the matrix
is vital for homogeneity in the bulk.
It should be of relatively low volatility under the
mass spectrometric vacuum conditions. A very volatile matrix compound may well give spectra but
they will be of a very short lifetime.
Ideally a matrix compound should not react chemically with the compound being analysed. If it does
react, it should be in a reproducible and predictable
In addition the matrix compound should be safe
to work with and cause as little contamination in
the mass spectrometer as possible. It should also be
stable and if possible of constant composition from
CCC-0306-042X/85/050191-06 $03.00
@ Wiley Heyden Ltd, 1985
batch to batch. This last point is particularly important where matrix compounds such as polyethylene
glycols are being used.
Table 1. Examples where the use of a cosolvent and/or
additive with glycerol has been shown to be
Glycerol (1)
Monellin, Insulin
Xanthane dyes
Cyanogenic glycosides
During preliminary work on development of the FAB
technique using charged particle sputter sources the
sample was deposited from solution and no matrix compound was present during the ionization process, resulting in spectra of a transient nature. It was noted,
however, that low vapour pressure liquids and oils gave
spectra that lasted for hours. Included in these samples
were Apiezon oils, Santovac 5 and Convalex 10.' Some
work was carried out with Santovac pump oil as a matrix'
but the first successful matrix compound was glycerol.
It is still the most widely used matrix compound.
The matrix cluster ions for glycerol are well documented, i.e. [93+(92),]+ in the positive ion mode and
[91+ (92)J in the negative ion mode. These ions are
routinely used for computer mass calibration in FAB
mass spectrometry. For higher mass calibration alkali
halides with or without glycerol can be used."
Molecules ionized from glycerol, as with most matrix
compounds, give [M + HI' ions under positive ion conditions and [M - HI- ions under negative ion conditions.
Glycerol adduct ions, e.g. [MH +glycerol]+ are
frequently observed. Lehmann et aL4 have also reported
the existence of [MH + 12]+ ions that increase with time.
They conclude from work with pentadeuteroglycerol
that this [MH+12]+ ion is due to formaldehyde produced in the glycerol resulting in Schiff base formation
as shown below.
Rose" has noted that on analysing some aldehydes
by FAB mass spectrometry, glycerol can form cyclic
metal complexes on the target. It has also been reported,'* in the study of trifunctional compounds with
boronic acids, that glycerol is reactive and forms cage
complexes. It was also shown that triethanolamine,
thioglycerol, 3-aminopropane-l,2-diol and 1,1,1tris( hydroxmethy1)ethane also formed cage complexes
with boronic acids.
Cationization occurs readily with glycerol as the
matrix compound with ions such as [M + Na]+ and [M +
K]+ being commonly observed. In addition cationized
glycerol ions are frequently observed. These cationized
species arise from the presence of trace amounts of
inorganic salt contaminants. The phenomenon of cationization can be used to enhance (see Table 1) spectral
intensity by the deliberate addition of alkali metals
halides, although suppression of the compound spectra
by the presence of alkali metal ions can occur. The
critical factors concerning whether alkali metal contami192 BIOMEDICAL MASS SPECTROMETRY, VOL. 12, NO. 5, 1985
DMSO, DMF, HCI, HCOOH 3,10,15,16,
Formic acid, alkali metal 22,23,24,26
salts, ammonium salts
Acetic acid
D M S O + l M HCI (3:l v/v)
nation becomes a problem or not are the concentration
of the alkali metal cation and the nature of the functional
groups in the molecule being analysed. Sample clean
up with high-performance liquid chromatography
(HPLC) or Sep-Pak cartridges has been recommended3
when alkali metal ions are a problem. With fractionated
fulvic acid sodium chloride addition has been shown'3
to enhance the abundance of the [ M + Na]+ species and
reduce fragmentation. The addition of alkali metal ions
can also be of use where there is some doubt concerning
the nature of the molecular species, e.g. [M + HIt or
[M + Na]+.
Often a cosolvent is required with glycerol. Various
alcohols, water, dimethylsulphoxide (DMSO) and
dimethylformamide (DMF) are routinely employed.
Some examples from the literature where the use of a
particular cosolvent or additive has been shown to be
beneficial are given in Table 1. Glycerol has been found
not to give useful FAB spectra from various (4,4'-disubstituted 2,2'-bipyridine)tetracarbonyl metal complexes
of molybdenum and tungsten but it has been shown that
if the complex is dissolved in D M F and then mixed with
glycerol good quality FAB spectra can be ~ b t a i n e d . ' ~
A useful table of matrix-associated ions observed in
FAB spectra when various additives are employed has
been p ~ b l i s h e d . ~
The use of acids for basic compounds and bases for
acidic compounds as aids to compound solubility has
been extensively reported (see Table 1). The dramatic
effect of acid concentration has been shown by Martin
and coworker^.^ No ions due to peptide derivatives were
observed in the molecular ion region with just glycerol
as the matrix, whereas addition of acetic acid increased
the abundance of the [M+ H]+ ion until it dominated
the spectrum. The effect of acid has also been noted
with the FAB spectra of d i g i t ~ n i n . ~
of the
glycerol matrix with either 0.1 M HC1 or 0.1 M acetic
acid had little effect on the spectra quality, whereas
addition of 0.1 M formic acid caused a marked increase
in the cationization of the molecular ion and thus in the
intensity of the spectrum.
The technique of derivatization on the target is also
well documented. Acetic anhydride has been used extensively to acetylate peptides** (often in combination with
deuteroacetylation) and methanol/hydrochloric acid
has been used to methylate carboxylic acids.3
Field29has discussed in detail FAB mass spectrometry
of glycerol with specific reference to radiation chemistry
and Chan and Cook3' have investigated the chemical
reactivity of glycerol as a mass spectrometric matrix.
Deuteroglycerol [(OZH)3glycerol]
Deuteroglycerol has been used for the quantitative determination of dehydroepiandrosterone s ~ l p h a t e ~ ' and
despite facile exchange of deuterium it has been used
to devise satisfactory standard curves and then to analyse
the sulphate in plasma extracts. Deuteroglycerol enables
interfering ions from the glycerol matrix to be shifted
in mass away from the ions of interest. The problem
with deuterium exchange that occurs with (02H),glycerol can be overcome by the use of monodeuteroglycerol.
Deuteroglycerol has also been used as a matrix to
enable the number of active hydrogens in both molecular
and fragment ions to be determined, e.g. with 1 7 - 0 - p - ~ kijanosylkijanolide where the protonated molecular ion
shifted from m / z 783 to m /z 787 when deuteroglycerol
was used in place of glycerol.20
Thioglycerol (2)
Thioglycerol(3-mercapto-1,2-propandiol) is used extensively as a FAB matrix compound despite its rather
unpleasant odour.
Lehmann et al? have studied in detail the FAB mass
spectrum of angiotensin with glycerol or thioglycerol as
the matrix compound. Both gave relatively stable and
long-lasting ion currents but thioglycerol was found to
produce up to 10 times greater ion current values.
Further studies indicated that a factor of between five
and 10 could be expected as the increase in sensitivity
when thioglycerol was used instead of glycerol for the
FAB mass spectrometry of angiotensin. These authors
pointed out the difficulty in correlating the positive effect
of thioglycerol with a defined physicochemical difference between thioglycerol and glycerol. Further experiments with various oligopeptides indicated that thioglycerol gave either as good as or better spectra than glycerol with this class of compound.
Thioglycerol has been used as the matrix compound
for the FAB mass spectral investigation of several
i n ~ u l i n and
s ~ ~other large pep tide^.^^ It was shown that
for insulin, acidification of the sample with glacial acetic
acid gave a marginally greater abundance for the [ M +
HIf ion, but the rate of decay of its abundance was
much faster than when thioglycerol on its own was used.
With adrenocorticotropic hormone ca. 1Y' O glacial acetic
acid in thioglycerol was found to be the matrix of choice,
whereas with melittin a mixture of thioglycerol and
tetraethyleneglycol containing 1 OO/ glacial acetic acid
gave the most intense spectra.
Quinoxaline antibiotics such as triostin A that fail to
give FAB spectra with glycerol, give intense spectra with
a 1: 1 (v/v) mixture of thioglycerol and digly~erol.~'
In work with g l ~ c a g o nthioglycerol
was utilized as
the matrix compound with trichloroacetic acid being
added to enhance production of the doubly charged
molecular ion.
Thioglycerol has also been used as the matrix compound for obtaining FAB mass spectra of several
coccidiostats, e.g. S e p t a m y ~ i nWith
. ~ ~ Kijanimicin, an
oligosaccharide containing macrotetronolide antibiotic,
substitution of glycerol by thioglycerol as matrix compound results in a substantial increase in the abundance
of the molecular ion species.20It has also been used to
obtain FAB mass spectra of the anthocyanins violanin
and platy~onin.~'
A variety of matrix compounds including glycerol, thioglycerol, tetramethylenesulphone and
triethanolamine have been tried during a study of the
FAB mass spectra of cationic technetium and iron complexes. It was found that signal intensity was greater
when thioglycerol was used.39 Rottschaefer and
Roberts4' have found that in the FAB mass spectral
analysis of gold-containing drugs the spectra were dominated by ion clusters containing sulphur and gold. The
use of thioglycerol with gold compounds is thus not to
be recommended.
Artefact formation when using thioglycerol has been
r e p ~ r t e d . Study
of the FAB mass spectra of deuteroporphyrin-6(7)-methyl ester, 7(6)-(histidine methyl ester)
indicated that in situ formation of the homologous
diamide had taken place.
Aminoglycerol (3)
Aminoglycerol has been successfully
for certain
glycopeptides where glycerol has been found to be
Thioglycolic acid (4)
Thioglycolic acid possesses the favourable features of
an acid and a reducing agent and has been used to
increase the sensitivity in the FAB mass spectra of certain
peptides such as angiotensin I.''
Polyethylene glycols and related compounds
Many ethylene glycol related compounds have been
used as matrix compounds. The use of polyethylene
glycol (PEG) as a calibration compound and matrix
compound for FAB mass spectrometry has been
described by several authors. In particular a mixture of
PEGS 200, 400 and 600 in a weight ratio of 1 :2 :4 has
been reported as a useful calibration mixture. An application of its use for the positive and particularly negative
ion spectra of di-, tri- and tetrasaccharides has been
L a t t i ~ n e rhas
~ ~ reported a detailed study of the FAB
mass spectra of polyglycols such as PEG 400, 600 and
1000 and PPG 41993 and 41994.With the more viscous
polymers, a small amount of thioglycerol was required
to make them more fluid.
Przybylski’ has used the oligopeptide Leu-Trp-MetArg-Phe-Ala as a test substance for a study of various
polyethylene glycols as matrix compounds. The lower
molecular weight PEGs yielded [M + H+] abundances
comparable with that obtained from glycerol with,
however, less matrix background in the lower mass
range. Possessing a lower vapour pressure than glycerol,
they gave spectra with long lifetimes. With the higher
molecular weight PEGs, drastically decreased sample
ion currents were observed, which was interpreted as
indicating the importance of sample mobility in the
Rose et ~ 1 have
. shown
~ ~ PEG 200 to be a very useful
matrix compound for glycosidic compounds particularly
for saponins and di- and trisaccharides. PEG 200 has
also been used as a matrix compound in a study of
reactions of boronic acids with triols and related
Rinehart4’ has suggested that other useful polyethylene glycols include tetragol HO(CH2CH20)4H and
tetraglyme CH30(CH2CH20)4CH3,while Williams
have suggested, in addition, teracol
HO(CH2CH2CH2CH20),H. A trigol-chloroform matrix
has been reported4’ as producing spectra showing
molecular and protonated molecular ions from 4-vinyl4-desethylchlorophyll.
Triton X100, an alkylphenylpolyethylene glycol, has
been shown to be a very useful matrix compound for
several porphyrins that did not give FAB mass spectra
with glycerol. Its use as a solubilizing agent with glycerol
(approx. 1 :99) has been reported. Excellent spectra of
chlorophyll A were obtained using this combination.’
Triethanolamine (5)
Triethanolamine has been shown to be a particularly
useful matrix compound giving improved spectra relative to glycerol with vitamin BL2and good spectra from
unsaturated fatty acids, ammonium oleate, various surfactants and certain pencillin derivative^.^^ It has also
been used for the analysis of anionic surfactants by FAB
mass s p e ~ t r o m e t r yWith
. ~ ~ ammonium lauryl ether sulphate (Sipex EA), (OCH2CH2),0SO;NH:, where there
was better solubility in triethanolamine than glycerol,
superior spectra were obtained particularly in the negative ion mode.
Work on the location of the double bond position in
unsaturated fatty acids by negative ion FAB tandem
mass spectrometry has employed triethanolamine as the
matrix compound, e.g. 9-octadecenoic acid.50
There have been several reports on the use of
triethanolamine for the FAB mass spectrometry of gangl i o ~ i d e s . ~The
’ - ~ variation
in the FAB spectra of GM3
ganglioside when glycerol, triethanolamine or a mixture
of triethanolamine and 1,1,3,3-tetramethyl urea were
used as the matrix compound has been r e p ~ r t e d . ~The
abundance of the molecular ion species was found to
be in the order glycerol < triethanolamine < mixture. In
addition the diagnostic fragment ions were easier to
differentiate from the matrix ions when the
triethanolamine-urea matrix was used. The authors pos194 BIOMEDICAL MASS SPECTROMETRY, VOL. 12, NO. 5, 1985
tulated that the role of the 1,1,3,3-tetramethylurea was
to disperse the sample in the triethanolamine by breaking
down or loosening the inter- or intramolecular hydrogen
bonds in the polyhydroxylated materials, rather than to
act as a matrix per se.
Diethanolamine, NH(CH2CH20H)2and triethanolamine have been used to produce FAB mass spectra of
folic acid.55 Signal enhancement was achieved by dissolving the acid in a saturated solution of TRIS
( NH2C(CH20H)J before mixing with the matrix.
The successful use of triethanolamine for the FAB
mass spectral analysis of steroid glycosides from an
oligoglycoside mixture isolated from the starfish Luidia
maculata has been reported.56
Diethanolamine (6)
Diethanolamine (DEA) has been used for the FAB mass
spectrometry of certain oli osaccharide containing
macrotetronolide antibiotics!
Particularly abundant
matrix adduct ions were observed corresponding to [ M +
The use of diethanolamine as a matrix compound in
secondary ion (SI) mass spectrometry has been
It was found to be particularly useful for
the study of complex oligosaccharides such as y-cyclodextrin. Abundant [M + DEAH]+ adduct ions were
observed and the spectra were less complex than those
obtained from triethanolamine, where adduct ions corresponding to [M + (HOCH2CH2),N=CHCH2OH]+ and
[ M + (HOCH2CH2)3NH]+were observed.
1,2,4-Butanetriol (7)
1,2,4-Butanetriol is less volatile than glycerol and has
been used successfully to obtain FAB mass spectra of
ph~sphatides.’~With lecithin the addition of trichloroacetic acid and phosphoric acid improved the
Thio-2,2’-bis(ethanol) (8)
Thio-2,2’-bis(ethanol) has been used as a matrix compound for various rhodium and osmium complexes
where glycerol was unsuitable.60
2,4-Di-tert-pentylphenol (DPP) (9)
C(CHJ2Cz H 5
Although used extensively by many groups, there are
few published references to the use of DPP as a matrix
A matrix consisting of DPP and acetonitrile has been
used to obtain good FAB mass spectra of Wilkinson’s
catalyst (10) and the bromine analogue (11):’
was run from a matrix of ammonium chloride and also
from sulpholane (12).
F U I ( P P ~ ~ ) ~ Rh(PPh,),Br
Crown ethers
Metal salts such as the chlorides, acetates and nitrates
of Li, K, Rh, Cs, Mg, La, Th, Co, etc. have been successfully run from an 18-Crown-6 matrix.62 It has also been
shown to be a good matrix for organometallic complexes. A matrix of 90% 18-Crown-6 and 10% tetraglyme (tetramethyleneglycoldimethylether) has been
used as the matrix of choice for a series of non-volatile
organometallic complexes especially rhodium, indium
and platinum complexes containing a cumulene
ligand.63 This mixture was developed by Minard and
Geoffrey64as an aprotic matrix of low volatility. It was
found that exposure to the matrix for periods of up to
several hours was useful for compounds not readily
soluble in it.
Miscellaneous matrix compounds
Meili and Seibl” have evaluated several organic liquids
for their effectiveness as matrix compounds. Folic acid
and a lipophilic diamide were used to study the various
liquids, as neither gave useful FAB mass spectra with
glycerol. With folic acid they found that diethanolamine
and triethanolamine were the most useful matrix compounds, especially when the acid was dissolved in
a saturated aqueous solution of TRIS before mixing
with the matrix compounds. Adduct ions were
observed including [M + DEAH]+ and [M + TRIS HI.+
With N, N, N’,N ’ -tetraisobutylcyclohexane-1,2-dicarboxamide, reasonable spectra were obtained from all
the matrix compounds tested, although the most suitable
was found to be TEC (triethyl citrate) especially when
hydrochloric acid was added. PEG was also found to
be useful, particularly as matrix suppression was good
and cleaner spectra were obtained. The other matrix
compounds tested were dibutyl succinate, 2-nitrophenyloctylether, linoleic acid, oleic acid and squalene.
These authors have also reported6’ the use of the ethyl
and butyl esters of citric acid, 2-nitrophenyloctylether
and benzoic acid benzyl ester as matrix compounds for
the FAB mass spectrometry of various corrin cobalt(II1)
complexes. They concluded that for the FAB mass spectral analysis of these compounds the use of a weakly
oxidizing matrix is to be recommended, as under these
conditions chemical degradation during the analysis can
be minimized.
Phosphoric acid has been used as a matrix compound
for the FAB mass spectral analysis of tetraphenylporphyrins.66
In a comparison of the FAB and SI mass spectra of
various pyridinium salts? the FAB mass spectrum of
sec-butyl-2,4,6-triphenylpyridinium tetrafluoroborate
The spectra obtained were similar with the cation (C)’
and a fragment ion due to (C-C,H,)+ dominating the
spectra. There were some differences, notably the presence of a (C-2)’ ion in the ammonium chloride spectrum, the presence of solvated cations containing one
or two molecules of sulpholane associated with the
cation in the sulpholane spectrum and the presence of
a cluster ion (2C+ A)+ also in the sulpholane spectrum.
Rinehart has also suggested sulpholane as a matrix
Dimethylsulphoxide has been used as the matrix in a
study of ion formation under FAB mass spectral conditions using N, N,N‘,N’-tetramethyl-1,Cbenzenediamine
as the sample.68
In a study of several aromatic hydrocarbons such as
anthracene and pyrene by FAB mass ~ p e c t r o m e t r ya~ ~
technical grade hydrocarbon fraction of boiling range
620-810 K was found to be a very suitable matrix.
A 3 : 1 mixture of dithiothreitol and dithioerythritol
(13)-the so-called ‘magic bullet’-has been found to
be a very useful matrix compound for many
organometallic corn pound^.^^
It can be seen from this review that an increasing number
of matrix compounds are being used to obtain FAB
mass spectra, although glycerol is still the most widely
used. It is anticipated that many other compounds will
be tried and found to be successful as matrix compounds
and it is hoped to report on these in the future. The
author would appreciate hearing from anyone who has
used matrix compounds not mentioned in this review
so that they can be included in a future review.
I would like to thank Michael Redrup, Gerald Risbridger and Keith
White‘ for help with this review and also the following FAB mass
spectrometry users who have sent me very useful information on the
matrix compounds they have used in their laboratories: Frank Crow
and Kenneth Tomer (Midwest Center for Mass Spectrometry), Anne
Dell (Imperial College), Simon Gaskell (Tenovus Institute), Ivor
Lewis (VG Analytical), Yuerg Meili (Swiss Federal Institute of Technology), Dave Millington (Duke University), John Monaghan (ICI),
Nico Nibbering (University of Amsterdam), Uwe Rapp (Finigan
MAT), Malcolm Rose (Sheffield Polytechnic), Ron Self (Food
Research Institute), John Spence (ICI), Yves Tondeur (National
Cancer Institute) and Dudley Williams (Cambridge).
I would like to express my thanks to Mrs Olga Stubbs for her
patience in typing this review, and to Mr A. E. Bird for his constructive
criticism of the paper.
Finally, I would like to thank the referee of this paper, whose
constructive criticism was of great value.
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Received 17 October 1984; accepted (revised) 1 February 1985
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