Atmospheric Pressure Photoionization mass spectrometry as the valuable method for
identification of polyisoprenoid alcohols
Magdalena Kania,*1 Karolina Skorupinska - Tudek, 2 Ewa Swiezewska 2 and Witold Danikiewicz 1
1.
2.
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01- 224 Warsaw,
Poland
Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Pawinskiego 5a, 02-106
Warsaw, Poland
Abstract
Rationale: The aim of this study was to prove that Atmospheric Pressure Photoionization
(APPI) is better suited for the MS analysis of polyisoprenoid alcohols comparing to the
commonly used Electrospray Ionization (ESI). The APPI method should make possible the
use of non-polar solvents without any additives required by ESI. An improvement of the
detection limit has been expected also.
Methods: The APPI- MS and ESI-MS spectra of polyisoprenoid alcohol standards were
acquired in the positive and in the negative ion modes, in methanol and hexane, using a
tandem mass spectrometer 4000 Q TRAP (Applied Biosystems Inc, USA) equipped with an
electrospray and an Atmospheric Pressure Photoionization ion sources and the triple
quadrupole/linear ion trap mass analyzer. The mixtures of polyisoprenoid alcohols were
analyzed by reversed phase liquid chromatography coupled with the mass spectrometer.
Results: In the positive ion mode peaks corresponding to [M + H  H2O]+ and [M + H]+ ions
were observed in the APPI-MS spectra of polyprenols and dolichols, respectively. In the
negative ion mode peaks corresponding to [M + O2]   and [M + Cl]  ions were observed for
both classes of polyisoprenoid alcohols. The detection limit of polyisoprenoid alcohols was
established at the level of 10 pg.
Conclusions: Atmospheric Pressure Photoionization (APPI) turned out to be a method of
choice for identification and quantitation of polyisoprenoid alcohols by mass spectrometry
using both polar and non-polar solvents. APPI, comparing to ESI, enables also greater
differentiation of polyprenols and dolichols concomitantly occurring in the natural samples.
Introduction
Atmospheric Pressure Photoionization (APPI) represents one of the soft ionization
techniques in mass spectrometry. Its main advantage is the possibility to analyze molecules
* Corresponding author: Magdalena Kania, Mass Spectrometry Group, Institute of Organic Chemistry, Polish Academy of
Sciences, Kasprzaka 44/52, 01- 224 Warsaw, Poland; e-mail: magdalena.kania@icho.edu.pl
1
which are poorly ionized in Electrospray Ionization (ESI) and Atmospheric Pressure
Chemical Ionization (APCI) methods. [1-3] APPI is dedicated for less polar and non-polar
compounds and gives an opportunity to analyze compounds in the positive and the negative
ion modes. This ionization method can be applied in a broad analytical area, for example to
the analysis of drugs and their metabolites, pesticides, lipids, flavonoids and steroids. [1, 2]
The aim of this work was to examine the application of APPI to identify and quantify
polyisoprenoid alcohols. These are linear five-carbon unit polymers which occur in all living
cells. Polyisoprenoid alcohol chains are built of 5-100 and more isoprenoid units and two
groups of these compounds are found in nature which differ by the structure of the terminal
isoprene unit: dolichols and polyprenols (Fig.1). Dolichols, mainly found in animal tissues,
yeast cells and some plant organs like roots, are α–saturated isoprenoid alcohols while
polyprenols occurring in plant photosynthetic tissues, wood, seeds, flowers and in bacterial
cells are the α–unsaturated counterparts. [4-6] It is also worth noting that in the cells mixtures
of dolichols are often accompanied by mixtures of polyprenols of the same chain-length
which increases the complexity of the analysis.
Fig. 1. Structures of polyprenols and dolichols
The biological role of polyisoprenoid alcohols is still examined. Polyprenols and
dolichols have been postulated to modulate the physical properties of model membranes, take
part in the transport of endoplasmic reticulum (ER) and vacuolar proteins while their
phosphates serve as cofactors of protein glycosylation and biosynthesis of glycosyl
phosphatidylinositol anchor in animal and probably also in plant cells. Dolichols are
postulated to protect the cellular membranes against peroxidation. [4]
Electrospray ionization has been used earlier to identify polyisoprenoid alcohols
in natural samples by mass spectrometry coupled with liquid chromatography (HPLC-MS).
[5-9] It requires, however, the addition of the salt solution to enhance the efficiency of the ESI
ionization of the analyte. Sodium or lithium salts can be added by means of the post-column
injection and/or directly to the sample solution in the positive ion mode of MS analysis. In the
negative ion mode Guan and Eichler showed that the presence of ammonium acetate in the
2
mobile phases caused the formation of adducts of polyprenoid alcohols with the acetate anion .
[8-10] Our current study shows successful application of APPI to analyze dolichols and
polyprenols in the standard solutions using direct mass spectrometry analysis as well as
HPLC-MS analysis. What is more no dopant was required in these experiments. Additionally,
APPI ionization methods permits more accurate identification of polyprenols and dolichols
concomitantly occurring in cells since different ions: [M + H  H2O]+ and [M+H]+ are
observed for polyprenols and dolichols, respectively, in the positive ion mode. The effects of
polar and non-polar solvents on the analyte ionization have been studied and the ion source
parameters were optimized. Finally, HPLC-APPI-MS analysis for the standard samples has
been carried out and compared with HPLC-ESI-MS.
Materials and methods
Chemicals
Samples of single dolichols (Dol-11, Dol-13, Dol-18), single polyprenols (Pren-11,
Pren-12, Pren-20), mixture of dolichols (from Dol-17 to Dol-21) and mixture of polyprenols
(Pren-9, Pren-11-23, Pren-25) were obtained from the Collection of Polyprenols (IBB PAN,
Warsaw, Poland). All solvents (HPLC grade) were from Merck (Darmstadt, Germany), water
was purified in Labconco purification system (Kansas City, MO, USA).
MS analysis
APPI-MS and ESI-MS experiments were performed using tandem mass spectrometer
4000 QTrap with PhotoSpray® and TurboIonSpray® sources, respectively, (Applied
Biosystems Inc, Foster City, CA, USA). The krypton lamp (10 eV) was employed in APPI
source. Nitrogen (Nitrogen Generator, Peak Scientific, Frankfurt, Germany) was used as the
nebulizing (GS1), the curtain (CUR) and the lamp gas (GS2). Analyses of polyisoprenoid
alcohols were carried out at declastering potential (DP) – 60 V and at entrance potential (EP)
– 10 V in both ion sources. The data were collected in the full-scan positive and negative ion
modes at a range of 400 – 1500 m/z in ESI-MS as well as APPI-MS measurements.
In APPI-MS analysis the following parameters depending on an investigated sample
were used: the temperature of the source and Ion Transfer Voltage (IS) in the range from 250
ºC to 350 ºC and 1000 - 1400 V, respectively, the lamp gas pressure – from 5 to 10 psi. In all
measurements the curtain gas was set 10 psi and the nebulizer gas – at 20 psi. Toluene was
tested as a dopant and it was delivered by a syringe pump (Harvard Apparatus, Holliston,
MA, USA) with a dopant flow rate at 30 µL/min.
3
In ESI-MS measurements the pressure of the curtain and the nebulizer gas were 20 psi
and 14 psi respectively. Ion Spray Voltage (IS) was set in the range from 5100 to 5500 V,
depending on an investigated sample.
HPLC-MS analysis
HPLC-APPI-MS (PhotoSpray®) MS and HPLC-ESI-MS (TurboIonSpray®) analyses
were carried out using a Prominence LC-20 (Shimadzu, Kyoto, Japan) HPLC system coupled
with tandem mass spectrometer 4000 Qtrap. HPLC separations were performed using the 4.6
x 250 mm Synergi Hydro-RP (4µm) (Phenomenex, Torrance, CA , USA) column. Solvent A
was a mixture of methanol/water/isopropanol (12/1/8, v/v) and solvent B – a mixture of
hexane and isopropanol (7/3, v/v). The linear gradient, from 100% phase A to 70 % phase B
in 50 minutes, was used. The injection volume was 5 µL/min, the flow rate - 1 mL/min and
the UV detector wavelength was set at 210 nm.
The standard sample of polyprenols was dissolved in a mixture of hexane and
isopropanol (1/2, v/v). In HPLC-ESI-MS and HPLC-APPI-MS analyses the concentration of
analyzed samples was 1 mg/mL.
Results and discussion
może dodać jedno zdanie ogólne:
While a few mass spectrometry techniques have been applied earlier to analyze
polyisoprenoids, including ESI-MS, Fast Atom Bombardment (FAB), Desorption Electron
Impact Ionization, Atmospheric Pressure Chemical Ionization nie pamietam dobrej pracy z
GC-MS lista dodatkowych referencji na koncu, the ESI technique seems to be best suited for
analysis of free, non-phosphorylated polyprenols and dolichols.
In the first step of investigations isoprenoid alcohols were analyzed by APPI-MS and
ESI-MS methods to compare these two ionization techniques. The following dolichols and
polyprenols, in hexane, were examined: Dol-11, Dol-13, Pren-11, Pren-12 and a mixture of
dolichols from Dol-17 to Dol-21. For APPI-MS the parameters of the ion source were
optimized and for shorter chain dolichols and polyprenols the optimal temperature of APPI
source and ion transfer voltage were found to be 250 ºC and 1000 V, respectively. For the
mixture of longer chain dolichols (Dol-17 to Dol-21) higher temperature of the ion source
and higher value of ion transfer voltage were required: 350 ºC and 1400 V, respectively.
Higher values of the source temperature and ion transfer voltage were probably required to
evaporate, ionize and transfer less volatile long chain dolichols. It was also necessary to
4
increase the pressure of the lamp gas, which protects the UV lamp against contamination, for
investigated mixture (from 5 to 10 psi).
In the second set of experiments the effect of dopant on ionization of dolichols and
polyprenols, dissolved in methanol and hexane as well, has been studied. Toluene which is
the most commonly used dopant has been tested. The recorded spectra showed that the dopant
was not required for the analysis of polyisoprenoid alcohols. Therefore, according to the
photoionization mechanism, it could be postulated that polyisoprenoids possess ionization
energy (IE) below 10 eV and could be ionized directly by photons. In the next step the
resulting radical cations can react with neutral solvent molecules giving protonated molecules
of polyisoprenoid alcohols. A moze to tak uprościć: Therefore, it could be postulated that
polyisoprenoids could be ionized directly by photons. [3, 11]
APPI technique makes possible the application of both polar and nonpolar solvents to
analyze compounds of interest. In this investigation methanol and hexane were used as the
representatives of these two classes of solvents. Identification of dolichols and polyprenols
was carried out in the positive and the negative ion modes. In APPI mass spectra recorded in
the positive ion mode, in both solvents protonated dolichol molecules were observed ([M +
H]+ , Fig. 2a) while APPI mass spectra of polyprenols are dominated by the peaks
corresponding to protonation and immediate loss of water from the protonated molecules ([M
+ H H2O], Fig. 2b).
+Q1: 0.083 to 0.334 min from Sample 6 (dol11_100ug_ml_1ul) of dolichol11_po...
Max. 3.8e6 cps.
[M +H]+ 769.7
3.8e6
3.5e6
In te n s ity , c p s
3.0e6
771.9
2.5e6
2.0e6
1.5e6
1.0e6
772.9
5.0e5
749.9 751.8
720
730
740
750
767.8
765.7
760
783.9 786.9
770
780
m/z, amu
790
815.8
801.9
800
810
820
830
5
Fig. 2a. APPI-MS spectrum of Dol-11 recorded in hexane in the positive ion mode, at the
following source parameters: IS - 1000 V, DP - 60 V and 250 ºC; pressure of the gases: GS1 –
20+Q1:
psi,0.083
GS2to –0.334
5 psi,
CUR
– 20
psi; the sample flow
rate – 200 µL/min.
min from
Sample
1 (pren12_100ug_ml_10ul)
of prenol12_w...
+
[M +H – H2O] 817.7
Max. 4.1e6 cps.
4.0e6
In te n s ity , c p s
3.5e6
3.0e6
2.5e6
819.9
2.0e6
1.5e6
835.9
1.0e6
820.8
5.0e5
807.8
770
780
790
800
815.8
810
836.9
829.8 833.8 837.9
820
830
840
m/z, amu
849.9
850
860
873.9
870
880
890
900
Fig. 2b. APPI-MS spectrum of Pren-12 recorded in methanol, in the positive ion mode, at the
following source parameters: IS - 1000 V, DP - 60 V and 250 ºC; pressure of the gases: GS1 –
20 psi, GS2 – 5 psi, CUR – 20 psi; the sample flow rate – 200 µL/min.
These results can be rationalized by the differences in the structures of dolichols and
polyprenols. The allylic character of polyprenol alcohol facilitates water elimination from the
preliminary formed protonated molecule yielding resonance-stabilized allylic cation:
-
This is not the case for dolichols which do not contain double bond in the α–residue.
In the negative ion mode [M + O2]   and [M + Cl]  ions are observed in the mass
spectra of both analyzed compounds, acquired either in methanol or hexane (Fig. 3a and 3b).
In the MS spectrum of Dol-11 recorded in hexane the peak at m/z 800.7 and 803.7
corresponds to [M + O2]  and [M + Cl], respectively (Fig. 3a).
6
-Q1: 0.083 to 0.334 min from Sample 7 (dol11_100ug_ml_1ul_ujemne) of dolich...
.
[M +O2] -
2.6e6
2.4e6
Max. 2.7e6 cps.
800.7
801.7
In te n s ity , c p s
2.2e6
[M +Cl]-
2.0e6
803.7
1.8e6
1.6e6
1.4e6
804.7
1.2e6
802.7
1.0e6
805.7
8.0e5
6.0e5
806.7
4.0e5
2.0e5
760
770
816.8
799.7 807.6
781.7
767.8
780
790
800
827.7
810
m/z, amu
820
832.7
830
841.7
840
849.7 855.8
850
860
Fig. 3a. APPI-MS spectrum of Dol-11 recorded in hexane, in the negative ion mode, at the
following source parameters: IS – (-1000) V, DP – (-60) V and 250 ºC; pressure of the gases:
GS1 – 20 psi, GS2 – 5 psi, CUR – 20 psi; the sample flow rate – 200 µL/min.
-Q1: 0.167 to 0.334 min from Sample 4 (pren12_100ug_ml_10ul) of prenol12_w...
[M +O2] - 866.7
6.0e5
Max. 6.0e5 cps.
.
5.5e5
In te n s ity , c p s
5.0e5
4.5e5
4.0e5
867.7
3.5e5
3.0e5
[M +Cl]-
2.5e5
869.6
2.0e5
868.7
1.5e5
1.0e5
871.7
833.7
5.0e4
803.5
800
814.7
810
820
835.6
830
847.6 849.7 865.7
840
850
878.6 877.7
860
870
m/z, amu
880
896.9
890
900
909.8
915.6
910
920
930
Fig. 3b. APPI-MS spectrum of Pren-12 recorded in methanol, in the negative ion mode, at the
following source parameters: IS – (-1000) V, DP – (-60) V and 250 ºC; pressure of the gases:
GS1 – 20 psi, GS2 – 5 psi, CUR – 20 psi; the sample flow rate – 200 µL/min.
7
In the next part of our study an effect of the solvent on ionization efficiency has been tested. It
was observed that the intensity of the signal recorded in methanol was significantly lower (e.
g. 5-fold for Pren-11) than that in hexane. This result is consistent with Ghislain et al. studies
where a self – doping effect of hexane has been postulated. [12] It can be also explained by
formation of methanol clusters which have lower ionization energy than the photons emitted
by the UV lamp. It was estimated earlier that at the flow rate of 50 µL/min the vapor
contained 99.98% of the methanol monomer (IE = 10.84 eV) and 0.02% of the methanol
dimer (IE = 9.47 eV). [1] Therefore charged dimer molecules can compete with dolichols and
polyprenols in the ionization process. Moreover, the solubility of these highly nonpolar
polyisoprenoid alcohols in hexane is higher than in methanol.
Analysis of the dolichol mixture by APPI-MS and ESI-MS methods (figures not
presented) showed the advantages of an application of the APPI source. In both ionization
techniques peaks corresponding to particular dolichols were observed. However, in APPI-MS
spectrum, recorded in hexane, significantly (5-fold) higher intensities of the peaks recorded
at 10-fold lower volume injection have been obtained.
This result prompted us to establish the smallest detectable amount of dolichols and
polyprenols using Selected Ion Monitoring (SIM) method. Short (Dol-13, Pren-12) and long
(Dol-18, Pren-20) chain polyisoprenoid alcohols have been studied. The ions at m/z 817.7,
905.8, 1246.8 and 1362.2 corresponding to Pren-12, Dol-13, Dol-18 and Dol-20, respectively,
were chosen for the experiments. The solutions, prepared in hexane, in the concentration
range from 1 pg/µl to 1 ng/µl were injected. Calibration curves obtained for a polyprenol and
a dolichol are presented in Fig. 4 and 5. In the ESI-MS analysis the linear detector response
was obtained in the range from 40 pg to 10 ng. [9] In the case of photoionization technique
the amount of 10 pg of each of the analyzed compounds was required for detection of both
polyisoprenoid alcohols by APPI technique. It is very likely that careful optimization of the
ion source parameters can lower the detection limit even further.
8
Pren-12
50000000
y = 4200,4x + 106
R2 = 0,9915
45000000
40000000
Peak Area
35000000
30000000
25000000
20000000
15000000
10000000
5000000
0
0
2000
4000
6000
8000
10000
12000
Sample Amount (pg)
Fig. 4. Calibration curve of Pren-12 (APPI in the positive ion mode, hexane used as a solvent).
Dol-20
5000000
y = 450,9x - 15103
R2 = 0,9986
4000000
Peak Area
3000000
2000000
1000000
0
0
2000
4000
6000
8000
10000
12000
-1000000
Sample Amount (pg)
Fig. 5. Calibration curve of Dol-20 (APPI in the positive ion mode, hexane used as a solvent).
9
Dolichols and polyprenols are usually identified as the mixtures of prenologues in
cells. Hence, APPI technique coupled with HPLC-MS method was employed for separation
and identification of the polyisoprenoid alcohols mixtures. The polyprenols mixture of Pren-9,
Pren-11 to Pren-23 and Pren-25 was analyzed under the same reversed phase
chromatographic conditions as for HPLC-ESI-MS. Identification of dolichols and polyprenols
by HPLC-APPI-MS method was performed in the positive and negative ion mode (Fig. 6).
Typical APPI-MS and ESI-MS chromatograms of polyprenols mixtures are shown in Figure 6
(a, b) and 7.
10
TIC of +Q1: from Sample 4 (pren _9_25 bez 24 w hex_IPA 200ul_500ul 5ul nastr...
Pren-11
17.32
2.6e8
30.99
Pren-12
2.4e8
a
Pren-18
19.50
29.65
2.2e8
In te n s ity , c p s
Max. 2.6e8 cps.
Pren-19
2.0e8
1.8e8
23.30
1.6e8
25.03
32.19
1.4e8
1.2e8
34.39
1.0e8
8.0e7
6.0e7
4.0e7
35.31
4.37
Pren-25
Pren-9
2.23
42.44 43.33
10.12
2.0e7
0.0
37.12
12.06
5
10
15
20
25
Time, min
30
35
40
TIC of -Q1: from Sample 6 (pren 9_25 5 ul nastrzyk w hex_IPA_200ul_500ul uje...
45.60
45
Max. 3.4e7 cps.
Pren-19
31.83
3.4e7
b
In te n s ity , c p s
3.0e7
Pren-18
2.5e7
30.56
2.0e7
Pren-11
1.5e7
19.12 24.52
32.97
26.17
Pren-23
1.0e7
5.0e60.62
0.0
36.08
3.93
4.70 11.64
5
10
16.29 17.21
15
40.25 46.02 47.14
20
25
Time, min
30
35
40
45
Fig. 6. APPI-MS chromatogram of a mixture of polyprenol standards solution (Pren-9, Pren-11-23,
Pren-25; 1 mg/mL) obtainedby HPLC-APPI-MS method, in the positive (a) and the negative (b)
ion mode. The separation was performed on Hydro RP using the 4.6 x 250 mm Synergi Hydro-RP
(4µm) (Phenomenex, Torrance, CA , USA) column. Solvent A was a mixture of
methanol/water/isopropanol (12/1/8, v/v) and solvent B – a mixture of hexane and isopropanol (7/3,
v/v). The linear gradient, from 100% phase A to 70 % phase B in 50 minutes, was used; the
injection volume was 5 µL, the flow rate - 1 mL/min.
11
TIC of +Q1: from Sample 3 (stand pren 9-11 11-23 25) of Magda IBB_Hydro_4_6...
3.2e9
3.0e9
Max. 3.2e9 cps.
2.25
4.28
In te n s ity , c p s
2.5e9
2.0e9
15.5616.26 17.81
12.39
1.5e9
21.84
28.01 31.63
33.79 41.80 42.93 44.64
10.22
Pren-11
Pren-11
Pren-10
1.0e9
5.0e8
0.0
5
10
15
20
25
Time, min
30
35
40
45
Fig. 7. ESI-MS chromatogram of a mixture of polyprenol standards solution obtained by HPLCESI-MS method, in the positive ion mode. The separation was performed on Hydro RP using the
4.6 x 250 mm Synergi Hydro-RP (4µm) (Phenomenex, Torrance, CA , USA) column. Solvent A
was a mixture of methanol/water/isopropanol (12/1/8, v/v) and solvent B – a mixture of hexane and
isopropanol (7/3, v/v). The linear gradient, from 100% phase A to 70 % phase B in 50 minutes, was
used; the injection volume was 10 µL, the flow rate - 1 mL/min.
In HPLC-ESI-MS, appearance of the analyzed compounds in MS chromatogram is
connected with the decrease of the total ion current (“negative” peaks). Since polyisoprenoids form
sodiated ions in ESI ionization technique, depleting of sodium ions by analyzed compounds in ESI
source can explain this phenomenon. [9] Such effect might result in underestimation of the longer
chain components of the mixture, cause incomplete identification of polyisoprenoids in the
analyzed samples and incorrect qualitative and quantitative results of the analysis. The peaks
corresponding to Pren-23 and Pren-25 are not visible in the ESI MS chromatogram (Fig. 7). The
previous studies showed that post-column supplementation with salt (e.g. ammonium acetate)
solution helps to enhance the intensity of isoprenoid peaks however this approach is not convenient
technically and causes some broadening of the chromatographic peaks. [8, 9]
In contrast to ESI-MS chromatogram, APPI-MS chromatogram of the analyzed mixture of
polyprenol standards showed the well-shaped peaks of 15 polyprenols (Fig. 6a). In the APPI-MS
spectra the intensity of peaks corresponding to the particular polyprenols is comparable with the
intensity of the same peaks in ESI-MS spectra despite using two times smaller amount of the
12
sample (approximate 5 µg) in HPLC-APPI-MS analysis. It is highly probable that signal intensity
obtained by HPLC-APPI-MS method can be further improved by modification of the mobile phase
composition. According to the results of direct APPI-MS analysis the presence of methanol and
water in solvent A can decrease the signal intensity (water tend to form clusters with methanol [1]).
Conclusions
Atmospheric Pressure Photoionization (APPI) seems to be a method of choice for successful
identification and quantitation of polyisoprenoid alcohols. This ionization technique permits an
effective ionization of analyzed polyisoprenoid alcohols in polar (i.e. methanol) and nonpolar (i.e.
hexane) solvents. The MS spectra of dolichols and polyprenols can be obtained in the positive and
the negative ion mode. The APPI gives an opportunity to analyze mixtures of dolichols and
polyprenols in a broad range of chromatographic systems. In contrast to the ESI ionization method,
APPI makes possible the use of the normal phase chromatography to separate polyisoprenoid
alcohols in natural samples. Moreover, taking the advantage from the different ions observed for
polyprenols and dolichol in the positive ion mode ([M  H2O]+ and [M + H]+, respectively), APPI
ionization method facilitates simultaneous identification of polyprenols and dolichols concomitantly
occurring in the natural polyisoprenoid mixtures. This point seems of special importance for the
further quantitative analysis of the MS spectra analogous to that applied earlier for the elucidation
of the polyisoprenoid biosynthetic pathway. [13]
Acknowledgments
This investigation was supported by a grant from the Polish Ministry of Science and
Education MNiSW NN303 311837 and a grant funded by the Polish National Cohesion Strategy
Innovative Economy UDA-POIG 01.03.01-14-036/09.
13
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